drm/imx: ldb: switch to drm_panel_bridge

drm/imx: ldb: switch to drm_panel_bridge

Defer panel handling to drm_panel_bridge, unifying codepaths for the
panel and bridge cases. The MFD_SYSCON symbol is moved to select to
prevent Kconfig symbol loops.

Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Tested-by: Chris Healy <cphealy@gmail.com>
Tested-by: Philipp Zabel <p.zabel@pengutronix.de> # on imx6q-nitrogen6x
Link: https://patchwork.freedesktop.org/patch/msgid/20240602-drm-imx-cleanup-v3-7-e549e2a43100@linaro.org
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>