drm/i915: Add intel_dotclock_calculate()

Extract the code to calculate the dotclock from the link clock and M/N
values into a new function from ironlake_crtc_clock_get().

The new function can be used to calculate the dotclock for both FDI and
DP cases.

Also simplify the code a bit along the way.

v2: Don't forget about non-pch encoders in ironlake_crtc_clock_get()

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>