drm/i915/glk: Don't allow 12 bpc when htotal is too big

Display workaround #1139 for Geminilake instructs us to restrict HDMI
to 8 bpc when htotal is greater than 5460. Otherwise, the pipe is unable
to generate a proper signal and is left in a state where corruption is
seen with other modes.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=100440


Cc: Shashank Sharma <shashank.sharma@intel.com>
Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Reviewed-by: Shashank Sharma <shashank.sharma@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170424104718.26448-1-ander.conselvan.de.oliveira@intel.com