drm/panel: himax-hx83121a: Fix incorrect error check for devm_drm_panel_alloc()

drm/panel: himax-hx83121a: Fix incorrect error check for devm_drm_panel_alloc()

Check devm_drm_panel_alloc() return value for ERR_PTR instead of NULL.
devm_drm_panel_alloc() returns an ERR_PTR on failure, never NULL. Using
a NULL check skips the error path and may cause a NULL pointer
dereference.

Fixes: a7c61963b727 ("drm/panel: Add Himax HX83121A panel driver")
Signed-off-by: Chen Ni <nichen@iscas.ac.cn>
Reviewed-by: Pengyu Luo <mitltlatltl@gmail.com>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://patch.msgid.link/20260327021728.647182-1-nichen@iscas.ac.cn