drm/i915/dsi: return -EBUSY instead of -1

Avoid using the incidental -EPERM. Also remove useless comment.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/37df1edc6d3745997cec2dfe41520d9f704e14b4.1633000838.git.jani.nikula@intel.com