drm/i915/dmabuf: Fix prime_mmap to work when using LMEM

The current implementation of i915 prime mmap only works when initializing
drm_i915_gem_object with shmem_region.
When using LMEM, drm_i915_gem_object is initialized with ttm_system_region.
In order to make prime mmap work even this case, when using LMEM
(when using ttm in i915), dma_buf_ops.mmap callback function calls
drm_gem_prime_mmap(). drm_gem_prime_mmap() of drm core calls internally
i915_gem_mmap() so that prime mmap can perform normally.
The fake offset is processed inside drm_gem_prime_mmap().

Testcase: igt/prime_mmap

Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
Reviewed-by: Nirmoy Das <nirmoy.das@intel.com>
Signed-off-by: Ramalingam C <ramalingam.c@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220225131316.1433515-3-gwan-gyeong.mun@intel.com

drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c

@@ -93,11 +93,15 @@ static void i915_gem_dmabuf_vunmap(struct dma_buf *dma_buf,
 static int i915_gem_dmabuf_mmap(struct dma_buf *dma_buf, struct vm_area_struct *vma)
 {
 	struct drm_i915_gem_object *obj = dma_buf_to_obj(dma_buf);
+	struct drm_i915_private *i915 = to_i915(obj->base.dev);
 	int ret;
 
 	if (obj->base.size < vma->vm_end - vma->vm_start)
 		return -EINVAL;
 
+	if (HAS_LMEM(i915))
+		return drm_gem_prime_mmap(&obj->base, vma);
+
 	if (!obj->base.filp)
 		return -ENODEV;