modesetting: Handle mixed VRR and non-VRR display setups better.

In a setup with both VRR capable and non-VRR capable displays,
it was so far inconsistent if the driver would allow use of
VRR support or not, as "is_connector_vrr_capable" was set to
whatever the capabilities of the last added drm output were.
Iow. the plugging order of monitors determined the outcome.

Fix this: Now if at least one display is VRR capable, the driver
will treat an X-Screen as capable for VRR, plugging order no
longer matters.

Tested with a dual-display setup with one VRR monitor and one
non-VRR monitor. This is also beneficial with the new Option
"AsyncFlipSecondaries".

When we are at it, also add some so far missing description of
the "VariableRefresh" driver option, copied from amdgpu-ddx.

Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>

hw/xfree86/drivers/modesetting/drmmode_display.c

@@ -3309,7 +3309,7 @@ drmmode_output_init(ScrnInfoPtr pScrn, drmmode_ptr drmmode, drmModeResPtr mode_r
         }
     }
 
-    ms->is_connector_vrr_capable =
+    ms->is_connector_vrr_capable |=
 	         drmmode_connector_check_vrr_capable(drmmode->fd,
                                                   drmmode_output->output_id);
     return 1;

hw/xfree86/drivers/modesetting/modesetting.man

@@ -71,6 +71,13 @@ One of \*qglamor\*q or \*qnone\*q.  Default: glamor.
 Enable DRI3 page flipping.  The default is
 .B on.
 .TP
+.BI "Option \*qVariableRefresh\*q \*q" boolean \*q
+Enables support for enabling variable refresh on the Screen's CRTCs
+when an suitable application is flipping via the Present extension.
+.br
+The default is
+.B off.
+.TP
 .BI "Option \*qAsyncFlipSecondaries\*q \*q" boolean \*q
 Use async flips for secondary video outputs on multi-display setups. If a screen
 has multiple displays attached and DRI3 page flipping is used, then only one of