drm/i915: Make intel_{mpllb,c10pll}_state_verify() safer

intel_{mpllb,c10pll}_state_verify() blows up if you call them
for a non-modeset/fastset commit on account of the relevant
connector not being part of the overall atomic state.
Currently the state checker only runs for modeset/fastset
commits, but for testing purposes it is sometimes desirable
to run it for other commits too. Check for modeset/fastset
in intel_{mpllb,c10pll}_state_verify() itself to make this safe.

v2: Give the new intel_c10pll_state_verify() the same treatment
    Add comment to explain why we do this

Reviewed-by: Jani Nikula <jani.nikula@intel.com> #v1
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230414190159.7904-1-ville.syrjala@linux.intel.com