meson: Only error building gallium video without libdrm when the platform is drm

Fixes: 3b265f61
       ("meson: gallium media state trackers require libdrm with x11")
Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/1878


Tested-by: Vinson Lee <vlee@freedesktop.org>
(cherry picked from commit 1481d054)