gma500: cdv_intel_hdmi: add missing include

drivers/gpu/drm/gma500/cdv_intel_hdmi.c:305:6: warning: no previous prototype for 'cdv_hdmi_init' [-Wmissing-prototypes]

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>