drm/uapi: Deprecate DRM_MODE_FLAG_PIXMUX

Reject any mode with DRM_MODE_FLAG_PIXMUX. We have no code that even
checks for this flag hence it can't possibly do any good.

Looks like this flag had something to do the the controller<->ramdac
interface with some ancient S3 graphics adapters. Why someone though
it would be a good idea to expose it directly to users I don't know.
And later on it got copied into the randr protocol and kms uapi.

Cc: Jose Abreu <Jose.Abreu@synopsys.com>
Cc: Adam Jackson <ajax@redhat.com>
Cc: Keith Packard <keithp@keithp.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171114183258.16976-4-ville.syrjala@linux.intel.com
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Jose Abreu <joabreu@synopsys.com>

include/drm/drm_modes.h

@@ -300,7 +300,7 @@ struct drm_display_mode {
 	 *  - DRM_MODE_FLAG_NCSYNC: composite sync is active low.
 	 *  - DRM_MODE_FLAG_HSKEW: hskew provided (not used?).
 	 *  - DRM_MODE_FLAG_BCAST: not used?
-	 *  - DRM_MODE_FLAG_PIXMUX: not used?
+	 *  - DRM_MODE_FLAG_PIXMUX: <deprecated>
 	 *  - DRM_MODE_FLAG_DBLCLK: double-clocked mode.
 	 *  - DRM_MODE_FLAG_CLKDIV2: half-clocked mode.
 	 *

include/uapi/drm/drm_mode.h

@@ -75,7 +75,7 @@ extern "C" {
 #define DRM_MODE_FLAG_NCSYNC			(1<<8)
 #define DRM_MODE_FLAG_HSKEW			(1<<9) /* hskew provided */
 #define DRM_MODE_FLAG_BCAST			(1<<10)
-#define DRM_MODE_FLAG_PIXMUX			(1<<11)
+#define DRM_MODE_FLAG_PIXMUX			(1<<11) /* deprecated */
 #define DRM_MODE_FLAG_DBLCLK			(1<<12)
 #define DRM_MODE_FLAG_CLKDIV2			(1<<13)
  /*
@@ -118,7 +118,6 @@ extern "C" {
 				 DRM_MODE_FLAG_NCSYNC |		\
 				 DRM_MODE_FLAG_HSKEW |		\
 				 DRM_MODE_FLAG_BCAST |		\
-				 DRM_MODE_FLAG_PIXMUX |		\
 				 DRM_MODE_FLAG_DBLCLK |		\
 				 DRM_MODE_FLAG_CLKDIV2 |	\
 				 DRM_MODE_FLAG_3D_MASK)