drm/amdgpu: Clear VRAM for DRM dumb_create buffers

drm/amdgpu: Clear VRAM for DRM dumb_create buffers

The dumb_create API isn't intended for high performance rendering
and it's more useful for userspace (ie. IGT) to have them precleared.

The bonus here is that we also won't needlessly leak whatever was
previously in VRAM, but it also probably wasn't sensitive if it was
going through this API.

Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>