Noticed this while moving all of the VESA backlight code in i915 over to
DRM helpers: it would appear that we calculate the frequency value we want
to write to DP_EDP_BACKLIGHT_FREQ_SET twice even though this value never
actually changes during runtime. So, let's simplify things by just caching
this value in intel_panel.backlight, and re-writing it as-needed.

Changes since v1:
* Wrap panel->backlight.edp.vesa.pwm_freq_pre_divider in
  DP_EDP_BACKLIGHT_FREQ_AUX_SET_CAP check - Jani
Signed-off-by: Lyude Paul <lyude@redhat.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Dave Airlie <airlied@gmail.com>
Cc: greg.depoire@gmail.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210514181504.565252-2-lyude@redhat.com