drm/panel: focaltech-ota7290b: Fix error check for devm_drm_panel_alloc()

drm/panel: focaltech-ota7290b: Fix error check for devm_drm_panel_alloc()

The devm_drm_panel_alloc() function returns an error pointer on failure,
not NULL. Fix the check to use IS_ERR() and return PTR_ERR() to
correctly handle allocation failures.

Fixes: 07853e954248 ("drm/panel: add driver for Waveshare 8.8" DSI TOUCH-A panel")
Signed-off-by: Chen Ni <nichen@iscas.ac.cn>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://patch.msgid.link/20260429080527.1456732-1-nichen@iscas.ac.cn