drm/panel: do not return negative error codes from drm_panel_get_modes()

drm/panel: do not return negative error codes from drm_panel_get_modes()

None of the callers of drm_panel_get_modes() expect it to return
negative error codes. Either they propagate the return value in their
struct drm_connector_helper_funcs .get_modes() hook (which is also not
supposed to return negative codes), or add it to other counts leading to
bogus values.

On the other hand, many of the struct drm_panel_funcs .get_modes() hooks
do return negative error codes, so handle them gracefully instead of
propagating further.

Return 0 for no modes, whatever the reason.

Cc: Neil Armstrong <neil.armstrong@linaro.org>
Cc: Jessica Zhang <quic_jesszhan@quicinc.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: stable@vger.kernel.org
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Reviewed-by: Jessica Zhang <quic_jesszhan@quicinc.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/79f559b72d8c493940417304e222a4b04dfa19c4.1709913674.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>