drm/fb: Don't expose mmio for fbdev emulation layer

For the fbdev api if the struct fb_var_screeninfo accel_flags field is set
to FB_ACCELF_TEXT then userland applications can not mmap the mmio region.
Since it is a bad idea for DRM drivers to expose the mmio region via the
fbdev layer we always set the accel_flags to prevent this. Please apply.

Signed-off-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>