drm: panel: ili9881: Correct symmetry on enable/disable return codes

ili9881c_enable is always returning 0.

ili9881c_disable was returning the error code from
mipi_dsi_dcs_set_display_off.
If non-zero, the drm_panel framework will leave the panel marked as
enabled, and not run the enable hook next time around. That isn't
helpful, particularly as we're expecting unprepare to disable
resets and regulators.

Change ili9881c_disable to match enable in always returning 0.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>