gpu: drm: xylon: Fixed NULL pointer dereference

Call drm_fb_helper_prepare() function for proper struct drm_fb_helper
initialization and remove manual drm_fb_helper_funcs pointer initialization.

Error log:
Unable to handle kernel NULL pointer dereference at virtual address 000001d0
pgd = 40004000
[000001d0] *pgd=00000000
Internal error: Oops - BUG: 5 [#1] PREEMPT SMP ARM
Modules linked in:
CPU: 0 PID: 6 Comm: kworker/u4:0 Not tainted 3.17.0
PC is at drm_fb_helper_single_add_all_connectors+0x10/0xa8
LR is at xylon_drm_fbdev_init+0x7c/0xfc

Signed-off-by: Davor Joja <davorjoja@logicbricks.com>
Tested-by: Christian Kohn <christian.kohn@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>