drm: mxsfb: Silence -EPROBE_DEFER while waiting for bridge

It can take multiple iterations until all components for an attached DSI
bridge are up leading to several:

[    3.796425] mxsfb 30320000.lcd-controller: Cannot connect bridge: -517
[    3.816952] mxsfb 30320000.lcd-controller: [drm:mxsfb_probe [mxsfb]] *ERROR* failed to attach bridge: -517

Silence this by checking for -EPROBE_DEFER and using dev_err_probe() so
we set a deferred reason in case a dependency fails to probe (which
quickly happens on small config/DT changes due to the rather long probe
chain which can include bridges, phys, panels, backights, leds, etc.).

This also removes the only DRM_DEV_ERROR() usage, the rest of the driver
uses dev_err().

Signed-off-by: Guido Günther <agx@sigxcpu.org>
Fixes: c42001e3 ("drm: mxsfb: Use drm_panel_bridge")
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/d5761eb871adde5464ba112b89d966568bc2ff6c.1608020391.git.agx@sigxcpu.org