drm/radeon: fix WREG32_OR macro setting bits in a register

This bug (introduced in 3.10) in WREG32_OR made
commit d3418eacad403033e95e49dc14afa37c2112c134
"drm/radeon/evergreen: setup HDMI before enabling it"
cause a regression. Sometimes audio over HDMI wasn't working, sometimes
display was corrupted.

This fixes:
https://bugzilla.kernel.org/show_bug.cgi?id=60687
https://bugzilla.kernel.org/show_bug.cgi?id=60709
https://bugs.freedesktop.org/show_bug.cgi?id=67767

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>