glamor: Make pixmap exportable from `gbm_bo_from_pixmap()`

If a pixmap is not exportable, `glamor_gbm_bo_from_pixmap()` would fail
and the modesettings driver would consequently fail to do its page flip,
which both prevents Present from working and also fill up the logs with
error messages such as:

  (EE) modeset(0): Failed to get GBM bo for flip to new front.
  (EE) modeset(0): present flip failed

Refactor the code so that `glamor_gbm_bo_from_pixmap()` takes care of
making the pixmap exportable.

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Signed-off-by: Yuxuan Shui <yshui@hadean.com>
See-also: !131
Closes: #68
Fixes: 86b2d874 "glamor: Reallocate pixmap storage without modifiers
       if necessary"
(cherry picked from commit 26fe29f4)