drm/xe: Fix an IS_ERR() vs NULL bug in xe_tile_alloc_vram()

The xe_vram_region_alloc() function returns NULL on error.  It never
returns error pointers.  Update the error checking to match.

Fixes: 4b0a5f5c ("drm/xe: Unify the initialization of VRAM regions")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Link: https://lore.kernel.org/r/5449065e-9758-4711-b706-78771c0753c4@sabinyo.mountain


Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>