compositor-drm: check capabilities before gbm modifiers setup

Commit 244244d1 ("compositor-drm: Use GBM modifier API")
force using of fb modifiers without runtime check.

As some modeset drivers don't support fb modifiers that
may lead to following error during weston initialization:
-------------------->8-------------------------
failed to create kms fb: No such file or directory
failed to get drm_fb for bo
-------------------->8-------------------------

Fix it by checking DRM_CAP_ADDFB2_MODIFIERS capability and rollback
to legacy (non gbm modifiers) codepath if required.

Fixes: 244244d1 ("compositor-drm: Use GBM modifier API")
Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Signed-off-by: Eugeniy Paltsev <PaltsevEvgeniy@gmail.com>
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