drm/amd/display: Use current connector state if NULL when checking bpc

[Why]
The old logic for checking which output depth to use relied on using
the current connector state rather than the new proposed state. This
was a problem when performing atomic commits since we weren't verifying
it against the incoming max_requested_bpc.

But switching this to only use the new state and not the current state
breaks filtering modes - it'll always assume that the maximum bpc
supported by the display is in use, which will cause certain modes
like 1440p@144Hz to be filtered even when using 8bpc.

[How]
Still use the connector->state if we aren't passed an explicit state.
This will respect the max_bpc the user currently has when filtering
modes.

Also remember to reset the default max_requested_bpc to 8 whenever
connector reset is called to retain old behavior when using the new
property.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=110845


Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>