ttm/dma: Remove the WARN() which is not useful.

. It was useful during development, but now on a production system
we can get this (if the user forgot to upload the firmware):

[drm] radeon: irq initialized.
[drm] GART: num cpu pages 131072, num gpu pages 131072
[drm] radeon: ib pool ready.
[drm] Loading SUMO Microcode
r600_cp: Failed to load firmware "radeon/SUMO_pfp.bin"
atl1c 0000:03:00.0: version 1.0.1.0-NAPI.213057] [drm:evergreen_startup] *ERROR* Failed to load firmware!
radeon 0000:00:01.0: disabling GPU acceleration
88] radeon 0000:00:01.0: ffff8801bb782400 unpin not necessary
------------[ cut here ]------------
WARNING: at /home/konrad/linux-linus/drivers/gpu/drm/ttm/ttm_page_alloc_dma.c:956 ttm_dma_unpopulate+0x79/0x300 [ttm]()
Hardware name: System Product Name
Modules linked in: e1000e atl1c radeon(+) ahci libahci libata scsi_mod fbcon tileblit font ttm bitblit softcursor drm_kms_helper wmi xen_blkfront xen_netfront fb_sys_fops sysimgblt sysfillrect syscopyarea xenfs xen_privcmd
Pid: 1600, comm: modprobe Not tainted 3.2.0-06100-ge343a895 #1
Call Trace:
 [<ffffffff8108973a>] warn_slowpath_common+0x7a/0xb0
 [<ffffffff81089785>] warn_slowpath_null+0x15/0x20
 [<ffffffffa0060309>] ttm_dma_unpopulate+0x79/0x300 [ttm]
 [<ffffffffa01341c0>] radeon_ttm_tt_unpopulate+0x120/0x130 [radeon]
 [<ffffffffa0056e0c>] ttm_tt_destroy+0x2c/0x70 [ttm]
 [<ffffffffa0057a4e>] ttm_bo_cleanup_memtype_use+0x3e/0x80 [ttm]
 [<ffffffffa00595a1>] ttm_bo_release+0x251/0x280 [ttm]
 [<ffffffffa0059610>] ttm_bo_unref+0x40/0x60 [ttm]
 [<ffffffffa0134d02>] radeon_bo_unref+0x42/0x80 [radeon]
 [<ffffffffa0186dfb>] radeon_sa_bo_manager_fini+0x6b/0x80 [radeon]
 [<ffffffffa0146b8f>] radeon_ib_pool_fini+0x6f/0x90 [radeon]
 [<ffffffffa014be49>] r100_ib_fini+0x19/0x20 [radeon]
 [<ffffffffa017b47e>] evergreen_init+0x1ee/0x2d0 [radeon]

The big WARN() has nothing to do with the culprit - which is that
the firmware was not loaded. So lets remove the WARN() from the TTM DMA code.
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/ttm/ttm_page_alloc_dma.c

@@ -952,10 +952,9 @@ void ttm_dma_unpopulate(struct ttm_dma_tt *ttm_dma, struct device *dev)
 
 	type = ttm_to_type(ttm->page_flags, ttm->caching_state);
 	pool = ttm_dma_find_pool(dev, type);
-	if (!pool) {
-		WARN_ON(!pool);
+	if (!pool)
 		return;
-	}
+
 	is_cached = (ttm_dma_find_pool(pool->dev,
 		     ttm_to_type(ttm->page_flags, tt_cached)) == pool);