drm/imx: imx8mp-ldb: return EPROBE_DEFER in probe

Return EPROBE_DEFER in the probe function if the LDB bridge is missing
some of its child bridges. The current implementation returns
EPROBE_DEFER in the bind function, which is incorrect because the kernel
assumes the driver is fully functional when bind is called. This results
in a deferal loop if native HDMI and the ldb bridge are enabled that
looks as follows:

dwhdmi-imx 32fd8000.hdmi: Detected HDMI TX controller v2.13a with HDCP (samsung_dw_hdmi_phy2)
dwhdmi-imx 32fd8000.hdmi: registered DesignWare HDMI I2C bus driver
imx-drm display-subsystem: bound imx-lcdifv3-crtc.0 (ops lcdifv3_crtc_ops)
imx-drm display-subsystem: bound imx-lcdifv3-crtc.1 (ops lcdifv3_crtc_ops)
dwhdmi-imx 32fd8000.hdmi: Detected HDMI TX controller v2.13a with HDCP (samsung_dw_hdmi_phy2)
dwhdmi-imx 32fd8000.hdmi: registered DesignWare HDMI I2C bus driver
...

The upstream imx8mp-ldb driver is a complete rewrite of the downstream
one and is most likely not affected because there is no bind:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/gpu/drm/bridge/fsl-ldb.c



Upstream-Status: Inappropriate [Other]
Signed-off-by: Stefan Eichenberger <stefan.eichenberger@toradex.com>