drm: Fix compilation error when CONFIG_DEBUG_FS is undefined

This patch fixes the following compilation error when CONFIG_DEBUG_FS is not defined.

../drivers/gpu/drm/drm_dp_helper.c: In function ‘drm_dp_aux_crc_work’:
../drivers/gpu/drm/drm_dp_helper.c:1029:13: error: ‘struct drm_crtc’ has no member named ‘crc’
../drivers/gpu/drm/drm_dp_helper.c:1031:12: error: ‘struct drm_crtc’ has no member named ‘crc’
  make[4]: *** [drivers/gpu/drm/drm_dp_helper.o] Error 1
  make[4]: *** Waiting for unfinished jobs....

Fixes: 79c1da7c ("drm/dp: add helpers for capture of frame CRCs")
Cc: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Cc: Sean Paul <seanpaul@chromium.org>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: David Airlie <airlied@linux.ie>
Cc: dri-devel@lists.freedesktop.org
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>