modesetting: Fix GBM objects leak when checking for flip

GBM objects were never destroyed after looking for format and
modifier compatibility when deciding whether flipping or copying
a presented pixmap.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=106106


Signed-off-by: Louis-Francis Ratté-Boulianne <lfrb@collabora.com>