backend-drm: set connector max bpc

"max bpc" property is meant for working around faulty sink hardware.
Normally it should be set to the maximum possible value so that the
kernel driver has full freedom to choose the link bpc without being
artificially forced to lower color precision.

The default value is 16 because that is a nice round number and more
than any link technology I've heard is using today which would be 12.

Also offer an API set the value, so that weston.ini could be used in the
next patch for sink workaround purposes.

Closes: https://gitlab.freedesktop.org/wayland/weston/-/issues/612

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>

include/libweston/backend-drm.h

@@ -78,6 +78,20 @@ struct weston_drm_output_api {
 	 */
 	void (*set_seat)(struct weston_output *output,
 			 const char *seat);
+
+	/** Set the "max bpc" KMS connector property
+	 *
+	 * 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.
+	 *
+	 * The value actually used in KMS is silently clamped to the range the
+	 * KMS driver claims to support. The default value is 16.
+	 *
+	 * This can be set only while the output is disabled.
+	 */
+	void (*set_max_bpc)(struct weston_output *output, unsigned max_bpc);
 };
 
 static inline const struct weston_drm_output_api *