drm/atomic-helper: Use bitwise or for filling a bitmask

drm/atomic-helper: Use bitwise or for filling a bitmask

Using += to set the bits in a mask looks funny. It works in this case
because we never set the same bit twice. But let's switch to |= to
make this look more regular.

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180615170734.2774-1-ville.syrjala@linux.intel.com
Reviewed-by: Eric Anholt <eric@anholt.net>