drm: ioremap balanced with iounmap for drivers/char/drm

ioremap must be balanced by an iounmap and failing to do so can result
in a memory leak.

Tested (compilation only) to make sure the files are compiling without
any warning/error due to new changes

Signed-off-by: Amol Lad <amol@verismonetworks.com>
Signed-off-by: Dave Airlie <airlied@linux.ie>