drm/radeon: update sarea copies of last_ variables on resume.

This fixes a regression reported in bug #12613.

[airlied: not I tweaked the patch slightly and fixed it by etienne did
all the hardwork so gets authorship]

Signed-off-by: etienne <etienne.basset@numericable.fr>
Signed-off-by: Dave Airlie <airlied@redhat.com>