drm: renesas: shmobile: Improve error handling

drm: renesas: shmobile: Improve error handling

Prepare for DT conversion, where panel probe can be deferred, by
streamlining error propagation and handling:
  - Use dev_err_probe() to avoid printing error messages in case of
    probe deferral,
  - Propagate errors where needed.

Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/6dfac76e5b1c7dda3f96801ce83845a4235e2ccd.1694767209.git.geert+renesas@glider.be