graphics/mesa: Simplify x11/wayland option

Simply use X11 for x11 support as it's done for all other ports.
Same for Wayland, just use WAYLAND.

While here always use xcb and xrandr for X11 support in mesa-dri as we
always build a vulkan driver (swrast).

Differential Revision: https://reviews.freebsd.org/D45743
Reviewed by: bapt
Sponsored by: Beckhoff Automation GmbH & Co. KG