bitmap: move bitmap_*_region() functions to bitmap.h

Now that bitmap_*_region() functions are implemented as thin wrappers
around others, it's worth to move them to the header, as it opens room
for compile-time optimizations.

CC: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
CC: Rasmus Villemoes <linux@rasmusvillemoes.dk>
CC: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Yury Norov <yury.norov@gmail.com>