gpu: amdgpu: fix broken amdgpu_dma_buf.c references

gpu: amdgpu: fix broken amdgpu_dma_buf.c references

This file was renamed, but docs weren't updated accordingly.

	WARNING: kernel-doc './scripts/kernel-doc -rst -enable-lineno -function PRIME Buffer Sharing ./drivers/gpu/drm/amd/amdgpu/amdgpu_prime.c' failed with return code 1
	WARNING: kernel-doc './scripts/kernel-doc -rst -enable-lineno -internal ./drivers/gpu/drm/amd/amdgpu/amdgpu_prime.c' failed with return code 2

Fixes: 2fbd6f94accdbb223a ("drm/amdgpu: rename amdgpu_prime.[ch] into amdgpu_dma_buf.[ch]")
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>