drm/i915/dpt: replace GEM_BUG_ON() with drm_WARN_ON()

Avoid using GEM_BUG_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/17e70eaf967bdfe99111cdbb3bcf6aa2f0b0e837.1694684044.git.jani.nikula@intel.com