drivers/gpu/drm/radeon/atom.c: fix warning

udelay() doesn't like 8-bit arguments:

drivers/gpu/drm/radeon/atom.c: In function 'atom_op_delay':
drivers/gpu/drm/radeon/atom.c:653: warning: comparison is always false due to limited range of data type

while we're there, use msleep() rather than open-coding it.

Cc: David Airlie <airlied@linux.ie>
Reviewed-by: Alex Deucher <alexdeucher@gmail.com>
Cc: Matt Turner <mattst88@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>