drm/panel: nt37801: Fix IS_ERR() vs NULL check in probe()

The devm_drm_panel_alloc() function returns error pointers, it doesn't
return NULL.  Update the check to match.

Fixes: 4fca6849 ("drm/panel: Add Novatek NT37801 panel driver")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Jessica Zhang <quic_jesszhan@quicinc.com>
Link: https://lore.kernel.org/r/aDCdn9r_ZAUTRpWn@stanley.mountain


Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>