drm/i915: Extract HAS_DOUBLE_BUFFERED_M_N()

We have a couple of places that want to make distinction between
double buffered M/N registers vs. the split M1/N1+M2/N2 registers.
Add a helper for that.

v2: Turn into a HAS_ macro (Jani)

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/20220907091057.11572-4-ville.syrjala@linux.intel.com