mpv: fix X11 logic after bump to 0.25.0

mpv: fix X11 logic after bump to 0.25.0

Between 0.24.0 and 0.25.0, mpv has changed its logic around the
detection/usage of X11 (in upstream commit
https://github.com/mpv-player/mpv/commit/927afa311d4a89b022305508acbf4e7325fe3928). Instead
of having fine grained option for x11, xext, xinerama, xscreensaver and
xrandr, mpv now requires all five libraries to be available in order to
enable X.org support.

Therefore, this commit adjusts the mpv packaging so that:

 1. All necessary X11 libraries are selected if BR2_PACKAGE_XORG7 is
    enabled. It would indeed be too cumbersome for the user to know that
    all of x11, xext, xinerama, xscreensaver and xrandr are necessary in
    order to have X.org support.

 2. mpv.mk is simplified by enabling X.org support when
    BR2_PACKAGE_XORG7 is enabled.

Fixes:

  http://autobuild.buildroot.net/results/b470cfba451ce9ba648e3a2f3ae08429db8bdee9/
  (--disable-xss was used to disable xscreensaver support, and this
  option no longer exists)

[Peter: reword X11 comment in .mk]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>