drm/i915: move pipe, pch and vblank enable to encoders on DDI platforms

To allow better flexibility for encoder specific code, push
intel_enable_pipe(), lpt_pch_enable() and intel_crtc_vblank_on() down to
the encoders from hsw_crtc_enable().

There's slight duplication, but also more clarity with the reduced
conditional statements.

Cc: Vandita Kulkarni <vandita.kulkarni@intel.com>
Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Reviewed-by: Vandita Kulkarni <vandita.kulkarni@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200128162850.8660-1-jani.nikula@intel.com