backends/drm: make modeset tests explicit

Instead of checking for properties needing a modeset, do atomic tests
with ALLOW_MODESET where it makes sense, and do a second atomic test
afterwards without ALLOW_MODESET to check if the modeset can be skipped.

This should ensure that KWin always does a modeset when it needs to do one,
 and not do a modeset when it's not necessary. Doing this also allows
reducing the complexity of the drm backend a bit.