drm/amd/display: fix mode scaling (RMX_.*)

As made mention of in commit 4a2df0d1 ("drm/amd/display: Fixed
non-native modes not lighting up"), we shouldn't call
drm_mode_set_crtcinfo() once the crtc timings have been decided. Since,
it can cause settings to be unintentionally overwritten. So, since
dm_state is never NULL now, we can use old_stream to determine if we
should call drm_mode_set_crtcinfo() because we only need to set the crtc
timing parameters for entirely new streams.

Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Fixes: bd49f190

 ("drm/amd/display: Always set crtcinfo from create_stream_for_sink")
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>