drm/usb: select USB_SUPPORT in Kconfig

DRM_USB selects USB. However, USB depends on USB_SUPPORT and USB_ARCH_HAS_HCD.
Thus, selecting USB_SUPPORT in Kconfig avoids the following warning
(detected when DisplayLink was selected using exynos4_defconfig):

warning: (MOUSE_APPLETOUCH && MOUSE_BCM5974 && MOUSE_SYNAPTICS_USB && JOYSTICK_XPAD && TABLET_USB_ACECAD && TABLET_USB_AIPTEK && TABLET_USB_HANWANG && TABLET_USB_KBTAB && TABLET_USB_WACOM && TOUCHSCREEN_USB_COMPOSITE && INPUT_ATI_REMOTE2 && INPUT_KEYSPAN_REMOTE && INPUT_POWERMATE && INPUT_YEALINK && INPUT_CM109 && RC_ATI_REMOTE && IR_IMON && IR_MCEUSB && IR_REDRAT3 && IR_STREAMZAP && IR_IGUANA && DRM_USB) selects USB which has unmet direct dependencies (USB_SUPPORT && USB_ARCH_HAS_HCD)

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>