drm/panel: jh057n00900: Add regulator support

Allow to specify regulators for vcc and iovcc. According to the data
sheet the panel wants vcc (2.8V) and iovcc (1.8V) and there's no startup
dependency between the two.

Signed-off-by: Guido Günther <agx@sigxcpu.org>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/f78611fb26329e50ec1533810fbb76562f2f4e48.1561542477.git.agx@sigxcpu.org