drm/i915: Adjust seamless_m_n flag behaviour

drm/i915: Adjust seamless_m_n flag behaviour

Make the seamless_m_n flag more like the update_pipe fastset
flag, ie. the flag will only be set if we need to do the seamless
M/N update, and in all other cases the flag is cleared. Also
rename the flag to update_m_n to make it more clear it's similar
to update_pipe.

I believe special casing seamless_m_n like this makes sense
as it also affects eg. vblank evasion. We can potentially avoid
some vblank evasion tricks, simplify some checks, and hopefully
will help with the VRR vs. M/N mess.

Cc: Manasi Navare <navaremanasi@chromium.org>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230901130440.2085-6-ville.syrjala@linux.intel.com
Reviewed-by: Manasi Navare <navaremanasi@chromium.org>