drm/i915: Get correct display clock on 945gm

This is according to Mobile Intel® 945 Express Chipset
Family datasheet.

Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Link: http://patchwork.freedesktop.org/patch/msgid/20170131235026.26003-1-arthur@aheymans.xyz


Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>