drm: Fix error path in drm_mode_page_flip_ioctl()

This fixes the error path for platforms that don't define the new
page_flip_target() hook.

Fixes: c229bfbb ("drm: Add page_flip_target CRTC hook v2")
Testcase: igt/kms_flip/basic-flip-vs-dpms
CC: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1473269025-16148-1-git-send-email-imre.deak@intel.com