drm/i915: Split i8xx_crtc_compute_clock()

Split a GEN2 specific version from i9xx_crtc_compute_clock(). With this
there is no need for i9xx_get_refclk() anymore, and the differences
between platforms become more obvious.

v2: Use i8xx as prefix instead of gen2. (Ville and Daniel)
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/1458653723-17951-1-git-send-email-ander.conselvan.de.oliveira@intel.com