drm/i915/fb: replace GEM_WARN_ON() with drm_WARN_ON()

Avoid using GEM_WARN_ON() in display code.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/593285450602c259b6985972d68511190c754bf5.1694684044.git.jani.nikula@intel.com