drm/amd/display: Only flush inst_fb if backdoor loading

[Why]
DAL resume from BACO time is longer if we always flush inst_fb

[How]
Check if backdoor loading to flush inst_fb

Signed-off-by: Alvin Lee <alvin.lee2@amd.com>
Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Eryk Brol <eryk.brol@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>