drm/radeon: fix typo in evergreen_mc_resume()

commit 695ddeb457584a602f2ba117d08ce37cf6ec1589 upstream.

Add missing index that may have led us to enabling
more crtcs than necessary.

May also fix:
https://bugs.freedesktop.org/show_bug.cgi?id=56139

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>