drm/amdgpu: Create helper to clear AMDGPU_GEM_CREATE_CPU_GTT_USWC

Move the logic to clear AMDGPU_GEM_CREATE_CPU_GTT_USWC in
amdgpu_bo_do_create into standalone helper so it can be reused
in other functions.

v4:
Switch to return bool.

v5: Fix typos.

Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Acked-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>