backend-drm: fix slightly wrong comment about allocating with no modifiers

In create_gbm_surface() we may allocate with no modifiers in the
following situations:

1. old GBM version, so HAVE_GBM_MODIFIERS is false;
2. the KMS driver does not support modifiers;
3. if allocating with modifiers failed, what can happen when the KMS
   display device supports modifiers but the GBM driver does not, e.g.
   the old i915 Mesa driver.

The comment was only stating the third situation, so add the other two.

Signed-off-by: Leandro Ribeiro <leandro.ribeiro@collabora.com>