drm/i915/dp: allow configuring eDP panel fitting scaling mode

LVDS allowed changing panel fitting scaling mode, while eDP didn't. Copied
relevant code from LVDS to eDP.

Signed-off-by: Yuly Novikov <ynovikov@chromium.org>
[Jani: use fitting mode in intel_panel, remove default mode change]
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Tested-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>