G2D: Now sunxi_x_g2d.c code does not require sunxi disp anymore

The sunxi_x_g2d.c file contains the midlayer code for hooking the
G2D optimized blit into xserver. But in fact it does not strictly
need to depend on anything sunxi specific.

So now we introduce a simple "blt2d_i" interface struct which
specifically provides a pointer to the accelerated blit function.
And just use this interface struct instead of the whole "sunxi_disp_t".
This allows to easily reuse the same code for other non-G2D or even
non-sunxi blit implementations in the future.

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