drm/panel: rocktech-jh057n00900: Rename the driver to st7703

This rename is done so that the driver matches the name of the
display controller and in preparation for adding support for more
panels to the driver.

This is just a basic file rename, with no code changes.

Signed-off-by: Ondrej Jirman <megous@megous.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Tested-by: Guido Günther <agx@sigxcpu.org>
Reviewed-by: Guido Günther <agx@sigxcpu.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200701162928.1638874-5-megous@megous.com