drm/sun4i: add missing header dependencies

We get 5 warnings when building kernel with W=1:
drivers/gpu/drm/sun4i/sun4i_framebuffer.c:33:23: warning: no previous prototype for 'sun4i_framebuffer_init' [-Wmissing-prototypes]
drivers/gpu/drm/sun4i/sun4i_framebuffer.c:47:6: warning: no previous prototype for 'sun4i_framebuffer_free' [-Wmissing-prototypes]
drivers/gpu/drm/sun4i/sun4i_rgb.c:202:5: warning: no previous prototype for 'sun4i_rgb_init' [-Wmissing-prototypes]
drivers/gpu/drm/sun4i/sun4i_dotclock.c:151:5: warning: no previous prototype for 'sun4i_dclk_create' [-Wmissing-prototypes]
drivers/gpu/drm/sun4i/sun4i_dotclock.c:186:5: warning: no previous prototype for 'sun4i_dclk_free' [-Wmissing-prototypes]

In fact, these functions are declared in
drivers/gpu/drm/sun4i/sun4i_framebuffer.h,
drivers/gpu/drm/sun4i/sun4i_rgb.h,
drivers/gpu/drm/sun4i/sun4i_dotclock.h,
so this patch adds missing header dependencies.

Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>