drm/amdgpu: Set GTT_USWC flag to enable freesync v2

This patch sets 'AMDGPU_GEM_CREATE_CPU_GTT_USWC' as input
parameter flag, during object creation of an imported DMA
buffer.

In absence of this flag:
1. Function amdgpu_display_supported_domains() doesn't add
   AMDGPU_GEM_DOMAIN_GTT as supported domain.
2. Due to which, Function amdgpu_display_user_framebuffer_create()
   refuses to create framebuffer for imported DMA buffers.
3. Due to which, AddFB() IOCTL fails.
4. Due to which, amdgpu_present_check_flip() check fails in DDX
5. Due to which DDX driver doesn't allow flips (goes to blitting)
6. Due to which setting Freesync/VRR property fails for PRIME buffers.

So, this patch finally enables Freesync with PRIME buffer offloading.

v2 (chk): instead of just checking the flag we copy it over if the
          exporter is an amdgpu device as well.

Signed-off-by: Shashank Sharma <shashank.sharma@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>