drm/i915/bdw: Broadwell has a max port clock of 300Mhz on HDMI

Just like HSW.

This means we can scan out a mode with a 300Mhz pixel clock with a depth
of 24 bits, but only a 200Mhz one with a 36bits depth.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>