soc: imx: gpc: Disable 6sl display power gating for ERR006287

The imx6sl chip errata document describes ERR006287 like this:

> Upon resuming from power gating, the modules in the display power domain
(eLCDIF, EPDC, PXP and SPDC) might fail to perform register reads
correctly.

> When the modules listed above are used, do not use power gating on the
display power domain.

Link: https://www.nxp.com/docs/en/errata/IMX6SLCE.pdf#page=62



Handle this in the safest possible way by keeping the DISP domain
always-on.

Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>