At the moment framebuffer driver does not have control over
mapping type for mmap(2). So when on ARM userland app mmaps
/dev/fb0 it gets default mapping which is write-back. So most of
the changes to fb region show up seconds later when respective
cache lines are evicted.

Proposed patch adds one more flag to fb_flags field that indicates
whether hardware driver need special type of mapping and adds one
more field with actual mapping type. bcm2835_fbd.c was used as an
example of the driver that requests VM_MEMATTR_WRITE_COMBINING.

Another possibility - always use VM_MEMATTR_WRITE_COMBINING, but
make sure that all platforms have it defined.