drm/i915/lspcon: Remove unused force change mode parameter

All callers asked for a forced change but the function ignored this
parameter. It doesn't seem to be necessary to force the change in any
case so let's just remove the parameter.

Cc: Shashank Sharma <shashank.sharma@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Shashank Sharma <shashank.sharma@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1479755707-29596-5-git-send-email-imre.deak@intel.com