drivers/dma: remove CONFIG_EXPERIMENTAL

This config item has not carried much meaning for a while now and is
almost always enabled by default. As agreed during the Linux kernel
summit, remove it.

CC: Vinod Koul <vinod.koul@intel.com>
CC: Dan Williams <djbw@fb.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Vinod Koul <vinod.koul@linux.intel.com>