drm: drop select of SLOW_WORK

slow-work got killed in commit 181a51f6. This means that since v2.6.36
there is no Kconfig symbol SLOW_WORK. Apparently selecting that symbol
is a nop. Drop that select.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Dave Airlie <airlied@redhat.com>