drm/amd/display: Look at firmware version to determine using dmub on dcn21

commit b1c61212 ("drm/amd/display: Fully switch to dmub for all dcn21
asics") switched over to using dmub on Renoir to fix Gitlab 1735, but this
implied a new dependency on newer firmware which might not be met on older
kernel versions.

Since sw_init runs before hw_init, there is an opportunity to determine
whether or not the firmware version is new to adjust the behavior.

Cc: Roman.Li@amd.com
BugLink: drm/amd#1772
BugLink: drm/amd#1735


Fixes: b1c61212 ("drm/amd/display: Fully switch to dmub for all dcn21 asics")
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Roman Li <Roman.Li@amd.com>
Change-Id: Ia2034361ac07fd221b1975f45f11e1b88e41f1c2