video: fbdev: fix sys_copyarea

The sys_copyarea() function performs the same operation as
cfb_copyarea() but using normal memory access instead of I/O
accessors.  Since the introduction of sys_copyarea(), there
have been two fixes to cfb_copyarea():

- 00a9d699 ("framebuffer: fix cfb_copyarea")
- 5b789da8 ("framebuffer: fix screen corruption when copying")

This patch incorporates the fixes into sys_copyarea() as well.

Signed-off-by: Mans Rullgard <mans@mansr.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>