drm/i915: Try to fix CRT port clock limits

drm/i915: Try to fix CRT port clock limits

LPT/WPT-H are limited to max 180 MHz CRT dotclock. Most other platforms
have a limit of 350 MHz. Supposedly gen3 and gen4 go up to 400 MHz.

VLV is a bit special since the docs are poor. Supposedly the DAC
would be good up to 355 MHz, but currently we limit the DPLL to
270 MHz, so we'll have to limit the port clock to the same unless
we change the DPLL limits.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1455738073-14502-7-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Imre Deak <imre.deak@intel.com>