Don't change the max_bpc connector prop if mode=current.

As things are, even when mode=current is specified on the .ini file,
a full modeset is needed (and done), which causes a very noticeable
screen blinking. That is because setting the max_bpc on a connector
needs full modesetting.
The idea here is that if mode=current on the .ini, no modesetting
should be done, so the current max_bpc is programmed into the
connector.
But if a custom max-bpc=... is specified, that will be used instead,
even if mode=current on the .ini

Fixes: https://gitlab.freedesktop.org/wayland/weston/-/issues/660

Signed-off-by: vanfanel <redwindwanderer@gmail.com>
(cherry picked from commit 3240ccc69d1488003c1cfc36d23750145d4f13f7)

include/libweston/backend-drm.h

@@ -84,7 +84,8 @@ struct weston_drm_output_api {
 	 * The property is used for working around faulty sink hardware like
 	 * monitors or media converters that mishandle the kernel driver
 	 * chosen bits-per-channel on the physical link. When having trouble,
-	 * try a lower value like 8.
+	 * try a lower value like 8. A value of 0 means that the current max
+	 * bpc will be reprogrammed.
 	 *
 	 * The value actually used in KMS is silently clamped to the range the
 	 * KMS driver claims to support. The default value is 16.