Reuse the already existing xserver framebuffer mapping for sunxi_disp_t

Avoid creating a new mapping because that's a waste of the virtual address
space. Also we are going to use this xserver framebuffer mapping address
for testing whether window backing pixmaps are allocated in the framebuffer
and can be accelerated by G2D.

Signed-off-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>