Fix build breakage when using older gbm lacking dmabuf import

Fix build breakage when using older gbm lacking dmabuf import

The buildbots discovered this issue on Ubuntu 14.04, which carries
libgbm 10.1.3-0ubuntu0.4.  The dmabuf changes need gbm 10.2, so it fails
during build like this:

  src/compositor-drm.c: In function ‘drm_output_prepare_overlay_view’:
  src/compositor-drm.c:984:10: error: variable ‘gbm_dmabuf’ has
  initializer but incomplete type
     struct gbm_import_fd_data gbm_dmabuf = {
            ^
  etc.

Proposed fix is to conditionalize the gbm fd import feature in
compositor-drm.

This fix was suggested by daniels.  I set up a synthetic test
environment to reproduce the issue as found by the buildbots and tweaked
the patch to get it to build both with and without gbm 10.2.

Signed-off-by: Bryce Harrington <bryce@osg.samsung.com>