Code cleanup.

Add local variable in several places to reference the MB mode
info structure. Currently this is usually accessed in the code as
x->e_mbd.mode_info_context->mbmi.* or in some places
xd->mode_info_context->mbmi.*

Resolved some uses of x-> for the MACROBLOCKD structure.

Rebased without dependency on motion reference experiment.

Change-Id: If6718276ee4f2ef131825d1524dfdb02a3793aed