drm/panel: remove return value of function drm_panel_add

The function "int drm_panel_add(struct drm_panel *panel)"
always returns 0, this return value is meaningless.
Also, there is no need to check return value which calls
"drm_panel_add and", error branch code will never run.

Signed-off-by: Bernard Zhao <bernard@vivo.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200801120216.8488-1-bernard@vivo.com