graphics/mesa-libs: Change default to use DRI3

Change the default mesa configuration to use DRI3 rather than the older DRI2
interface. This should improve performance somewhat, and alleviates the need
for the FIXDRM option in x11-servers/xorg-server.

Remove the FIXDRM option from x11-servers/xorg-server.

Add an UPDATING entry for the change.

For users of graphics/drm-legacy-kmod or the base graphics drivers, this might
cause regressions. If you experience problems when running OpenGL applications
please force the use of the DRI2 backend by setting the LIBGL_DRI3_DISABLE
environment variable to 1 before starting any OpenGL application. This is
easiest done by adding it to your shell startup file or .xinitrc.

Add UPDATING entry for xorg-server, detailing the change of device
configuration backend.

PR: 196678, 244306 (for tracking)