drm/i915: refactor pll code out into intel_dpll.c

This pulls a large chunk of the pll calculation code out of
intel_display.c to a new file.

One function makes sense to be an inline, otherwise this
is pretty much a straight copy cover. Also all the
remaining hooks for g45 and older end up the same now.

Signed-off-by: Dave Airlie <airlied@redhat.com>
[Jani: cleaned up intel_dpll.h a bit, de-duped intel_panel_use_ssc().]
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/74b58e0572858b5d1734818ca594a23040d7d44f.1610622609.git.jani.nikula@intel.com