arm64: dts: imx8mm-venice-gw7903: disable disp_blk_ctrl

The GW7903 does not connect the VDD_MIPI power rails thus MIPI is
disabled. However we must also disable disp_blk_ctrl as it uses the
pgc_mipi power domain and without it being disabled imx8m-blk-ctrl will
fail to probe:
imx8m-blk-ctrl 32e28000.blk-ctrl: error -ETIMEDOUT: failed to attach power domain "mipi-dsi"
imx8m-blk-ctrl: probe of 32e28000.blk-ctrl failed with error -110

Fixes: a72ba91e

 ("arm64: dts: imx: Add i.mx8mm Gateworks gw7903 dts support")
Signed-off-by: Tim Harvey <tharvey@gateworks.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>