drm/i915: Fix intel_crtc_mode_get() mode clock

i9xx_crtc_clock_get() no longer populates adjusted_mode.clock, so we
must get the pixel clock from port_clock in intel_crtc_mode_get().

This bug caused Chris's 845g machine to lockup during boot, and it
was introduced in:

 commit 18442d08
 Author: Ville Syrjälä <ville.syrjala@linux.intel.com>
 Date:   Fri Sep 13 16:00:08 2013 +0300

    drm/i915: Fix port_clock and adjusted_mode.clock readout all over

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=69713


Tested-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>