drm/amdgpu: Nerf buff

buff --> buf. Essentially buffer abbreviates to
buf, remove 1/2 of it, or just the iron part, as
opposed to just the Er,

Cc: Alexander Deucher <Alexander.Deucher@amd.com>
Cc: Andrey Grodzovsky <Andrey.Grodzovsky@amd.com>
Signed-off-by: Luben Tuikov <luben.tuikov@amd.com>
Reviewed-by: Alexander Deucher <Alexander.Deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>