drm/i915: make lspcon_get_current_mode static

Fixes sparse warning:

drivers/gpu/drm/i915/intel_lspcon.c:30:22: warning: symbol
'lspcon_get_current_mode' was not declared. Should it be static?

Fixes: dbe9e61b ("drm/i915: Add lspcon support for I915 driver")
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Shashank Sharma <shashank.sharma@intel.com>
Reviewed-by: Shashank Sharma <shashank.sharma@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1476789711-19697-2-git-send-email-jani.nikula@intel.com