drm/i915/display: split out modeset verification code

Add new file intel_modeset_verify.c for high level modeset verification
code to declutter intel_display.h. The new file is supposed to be about
crtc/encoder/connector verification; the state verification for very
specific functionality such as plls or wm should be placed next to the
code it verifies.

Fix some minor checkpatch issues while at it.

v2: Rebase

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/b9b47c14316a9edb772a8b8f934eabe7e928dd76.1655372759.git.jani.nikula@intel.com