gbm: gbm_bo_get_handle_for_plane fallback to nonplanar handle

gbm: gbm_bo_get_handle_for_plane fallback to nonplanar handle

Commit f9567ab435217a72cbae628336ead84dc0b2a803 (gbm: Export a getter for per
plane handles) contains an API version check that fails on i915 (API version 7
vs. check for minimum API version 13). Any client that migrates to the planar
API will start failing on i915 (see https://gitlab.gnome.org/GNOME/mutter/issues/127
for mutter, and https://bugs.freedesktop.org/show_bug.cgi?id=108487 for weston).

This commit adds a fallback for plane 0 when the API check fails and returns the
non-planar handle in this scenario, making the call equivalent to
gbm_bo_get_handle(). This is enough for weston 6.0.0 to start working again on an
i915 system.

Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=108487
Signed-off-by: Alex Villacís Lasso <a_villacis@palosanto.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>