hecubafb: use fb_sys_read()

Since hecubafb's framebuffer is vmalloc'ed, use fb_sys_read().

Signed-off-by: Antonino Daplas <adaplas@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>