drm/i915/panel: mass rename functions to have intel_panel_ prefix

Follow the usual naming conventions. Also pull HAS_GMCH() check to
intel_panel_fitting(). No functional changes.

Cc: Lyude Paul <lyude@redhat.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/9ff6e42e377bdb0c9349f50d9ea79671059633c7.1629888677.git.jani.nikula@intel.com