drm/i915: Introduce intel_panel_{fixed,downclock}_mode()

drm/i915: Introduce intel_panel_{fixed,downclock}_mode()

Abstract away the details on where we store the fixed/downclock
modes, and also how we select them. Will be useful for static
DRRS (aka. allowing the user to select the refresh rate for the
panel).

We pass in the user requested mode to intel_panel_fixed_mode()
so that in the future it may try to match the refresh rate.
And intel_panel_downclock_mode() gets passed the adjusted_mode
we actually chose to use so that it may find a suitable lower
resresh rate variant.

v2: Hook it up for all encoders
    s/fixed_mode/adjusted_mode/ in intel_panel_downclock_mode() (Jani)
    Elaborate on the choice or arguments for the functions (Jani)

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220311172428.14685-7-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>