drm/i915: Split g4x_crtc_compute_clock()

Split a G4X specific version from i9xx_crtc_compute_clock(). With this
the differences between platforms become more obvious.

Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1458576016-30348-15-git-send-email-ander.conselvan.de.oliveira@intel.com