package/weston: drop GBM dependency from DRM option

upstream advocated that DRM backend can be built without GBM:
https://gitlab.freedesktop.org/wayland/weston/-/merge_requests/791


so drop dependencies on
BR2_PACKAGE_MESA3D_OPENGL_EGL || BR2_PACKAGE_IMX_GPU_VIV_OUTPUT_WL
to allow DRM to be the default backend when fbdev will be removed in the
follow-up patch

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>