drm/radeon/kms: add sanity check to wptr.

If we resume in a bad way, we'll get 0xffffffff in wptr, and then
oops with no console. This just adds a sanity check so that we can
avoid the oops and hopefully get more details out of people's systems.
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/radeon/r100.c

@@ -744,6 +744,8 @@ int r100_cp_init(struct radeon_device *rdev, unsigned ring_size)
 	udelay(10);
 	rdev->cp.rptr = RREG32(RADEON_CP_RB_RPTR);
 	rdev->cp.wptr = RREG32(RADEON_CP_RB_WPTR);
+	/* protect against crazy HW on resume */
+	rdev->cp.wptr &= rdev->cp.ptr_mask;
 	/* Set cp mode to bus mastering & enable cp*/
 	WREG32(RADEON_CP_CSQ_MODE,
 	       REG_SET(RADEON_INDIRECT2_START, indirect2_start) |