blackfin video driver: fix bug when opening/reading/mmaping BF54x and BF52x framebuffer simultaneously

http://blackfin.uclinux.org/gf/project/uclinux-dist/tracker/?action=TrackerItemEdit&tracker_item_id=3974

opening/reading/mmaping BF54x and BF52x framebuffer simultaneously triggers
BUG: failure at mm/nommu.c:470/add_nommu_vma() Add VM_SHARED to the default
vm_flags
Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Bryan Wu <cooloney@kernel.org>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: "Antonino A. Daplas" <adaplas@pol.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>