Enabling SEPARATE_GLOBAL_MOTION will remove the ability for
a block that uses zeromv with global motion to pick warped_causal
or obmc_causal as the motion mode. When this is enabled there is:

0.05% drop on lowres for global + warped enabled
0.15% drop on midres for global + warped enabled

0.12% drop on lowres with global + motion var enabled
0.07% drop on midres with global + motion var enabled
No performance change for global, warped, or motion var individually.

Change-Id: Idbfb8dd7a93da14902438504b06a08e5212e48cb