drm/amdgpu: rename amdgpu_prime.[ch] into amdgpu_dma_buf.[ch]

We are getting a dma-buf implementation completely separate from drm prime,
so rename the files now and cleanup the code a bit.

No functional change.

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>