drm/i915: Split intel_cpu_transcoder_set_m_n() into M1/N1 vs. M2/N2 variants

drm/i915: Split intel_cpu_transcoder_set_m_n() into M1/N1 vs. M2/N2 variants

Make things a bit more explicit by splitting
intel_cpu_transcoder_set_m_n() into separate variants for M1/N1 vs.
M2/N2. Makes the DRRS M/N programming at least more obvious.

Note that for the MST and DRRS cases we don't need to call the
M2/N2 variant at all since the transcoders that support those
do not have the M2/N2 registers.

Same could be said for i9xx_crtc_enable() but I want to do a
higher level code sharing between that valleyview_crtc_enable()
later in which case we do need the M2/N2 variant. This is also
why I keep the transcoder_has_m2_n2() in intel_cpu_transcoder_set_m2_n2()
so the caller doesn't have necessarily care what the chosen
transcoder supports.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220128103757.22461-5-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>