drm/i915/bios: Extract get_panel_type()

Pull the code to determine the panel type into its own set of
sane functions.

v2: rebase

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220504150440.13748-7-ville.syrjala@linux.intel.com