drm/i915: Introduce intel_panel_compute_config()

Let's introduce a compute_config() helper for fixed mode panels.
For now all it does is the fixed_mode->adjusted_mode copy.

Note that with sDVO we have to ask the external encoder chip
to spit out our actual display timings for us, so the fixed_mode
to adjusted_mode copy done by intel_panel_compute_config() is
redundant, but we still want to use it to do other checks for us
later. We'll be fine so long as we only call it before
intel_sdvo_get_preferred_input_mode() overwrites adjusted_mode
with the timings from the encoder.

v2: Use intel_panel_compute_config() with sDVO

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210927185207.13620-1-ville.syrjala@linux.intel.com


Reviewed-by: Jani Nikula <jani.nikula@intel.com>