drm/i915: Grab the pixel clock from adjusted_mode not requested_mode

i9xx_set_pipeconf() attempts to get the current pixel clock from
requested_mode. requested_mode.clock may be totally bogus, so the
clock should come from adjusted_mode.

v2: Dropped the intel_compute_config() hunk due to killing of the
    INTEL_FDI_FREQ check

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>