drm/i915: Check pixel clock in ns2501 mode_valid hook

The vbt on my Fujitsu-Siemens Lifebook S6010 provides two 800x600 modes,
60Hz and 56Hz. The magic register values we have correspond to the 60Hz
mode, and as I don't know how one would trick the VGA BIOS to set up
the 56Hz mode we can't get the magic values for the orther mode. So
when checking whether a mode is valid also check the pixel clock so that
we filter out the 56Hz variant.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Thomas Richter <richter@rus.uni-stuttgart.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>