drm/i915: Remove intel_panel_detect()

With neither LVDS or eDP no longer using intel_panel_detect() we can
kill it, and the accompanying modparam.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180717174216.22252-3-ville.syrjala@linux.intel.com
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>