drm/radeon: fix hdmi mode enable on RS600/RS690/RS740

commit dcb852905772416e322536ced5cb3c796d176af5 upstream.

These chips were previously skipped since they are
pre-R600.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
[wml: Backported to 3.4:
- adjust context
- no !ASIC_IS_DCE3(rdev)]
Signed-off-by: Weng Meiling <wengmeiling.weng@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>