drm/amdgpu: Initialise drm_gem_object_funcs for imported BOs

For BOs imported from outside of amdgpu, setting of amdgpu_gem_object_funcs
was missing in amdgpu_dma_buf_create_obj. Fix by refactoring BO creation
and amdgpu_gem_object_funcs setting into single function called
from both code paths.

Fixes: d693def4

 ("drm: Remove obsolete GEM and PRIME callbacks from struct drm_driver")

v2: Use use amdgpu_gem_object_create() directly
v3: fix warning

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>