drm/amdgpu: cleanup amdgpu_ttm_copy_mem_to_mem and amdgpu_map_buffer v2

Cleanup amdgpu_ttm_copy_mem_to_mem by using fewer variables
for the same value.

Rename amdgpu_map_buffer to amdgpu_ttm_map_buffer, move it
to avoid the forward decleration, cleanup by moving the map
decission into the function and add some documentation.

No functional change.

v2: add some more cleanup suggested by Felix

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Tested-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>