drm/amd/display: allocate fbc buffer in AMDGPU_GEM_DOMAIN_GTT

Currently the FBC buffer is allocated in VRAM, since VRAM usage is
dedicatedly for scanouts, by allocating FBC back buffer in GTT
shall help in conserving VRAM for other purposes.

Signed-off-by: Shirish S <shirish.s@amd.com>
Reviewed-by: Charlene Liu <Charlene.Liu@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>