Code of dma_alloc_coherent() and dma_alloc_writecombine() is almost
identical with exception of the flags passed to undelying call to
__remap_range(). Move commong code into a shared subroutine and
convert both functions to use it.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>