drm/amdgpu: Only disable prefer_shadow on hawaii

We changed it for all asics due to a hibernation regression
on hawaii, but the workaround breaks suspend on a polaris12.
Just disable it for hawaii.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=216119
Fixes: 3a4b1cc2

 ("drm/amdgpu/display: disable prefer_shadow for generic fb helpers")
Reviewed-and-tested-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org