drm/amdgpu: update the domain flags for dumb buffer creation

After switching to generic framebuffer framework, we rely on the
->dumb_create routine for frame buffer creation. However, the
different domain flags used are not optimal. Add the contiguous
flag to directly allocate the scanout BO as one linear buffer.

Fixes: 087451f3 ("drm/amdgpu: use generic fb helpers instead of setting up AMD own's.")

Signed-off-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Guchun Chen <guchun.chen@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>