gxt4500: enable panning

The driver implements pan_display but the corresponding flags are not set.

Add FBINFO_HWACCEL_XPAN and FBINFO_HWACCEL_YPAN to flags to allow HW
accelerated panning (for fast scrolling).

Signed-off-by: Ondrej Zary <linux@rainbow-software.org>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>