[CORE] vio: Add vmm_vdisplay_get_pixeldata() API

For implementing "vscreen hard_bind", we need a vmm_vdisplay API
which will enable us to get format, dimensions, and host address
of guest virtual display. This API will help us to directly point
host frame buffer device to guest frame buffer.

This patch adds vmm_vdisplay_get_pixeldata() API for the above
purpose and also implements gfx_pixeldata() callback for pl110
emulator.

Signed-off-by: Anup Patel <anup@brainfault.org>