drm/i915: Eliminate port sync copy pasta

Remove the copy pasted port sync crtc enable functions and instead
just split the normal function into the two parts we need.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200313164831.5980-10-ville.syrjala@linux.intel.com


Reviewed-by: José Roberto de Souza <jose.souza@intel.com>