drm/gma500: remove unnecessary config_enabled() guard

Commit d112a816 ("gma500/cdv: Add eDP support") replaced the
code inside this if-conditional with gma_backlight_set(), which
becomes a nop stub if CONFIG_BACKLIGHT_CLASS_DEVICE is disabled.
So, there is no need to guard the caller with config_enabled().

Note:
This is one of remaining TODOs to deprecate config_enabled() macro.
Refer to commit 97f2645f ("tree-wide: replace config_enabled()
with IS_ENABLED()").

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1471970574-23906-1-git-send-email-yamada.masahiro@socionext.com