drm/amdgpu: Nerf "buff" to "buf"

Buffer is abbreviated "buf" (buf-fer), not "buff" (buff-er).
This is consistent with the rest of the kernel code.

Cc: Kent Russell <kent.russell@amd.com>
Cc: Alex Deucher <Alexander.Deucher@amd.com>
Signed-off-by: Luben Tuikov <luben.tuikov@amd.com>
Acked-by: Harish Kasiviswanathan <Harish.Kasiviswanathan@amd.com>
Reviewed-by: Kent Russell <kent.russell@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>