drm/msm/sde: enable master physical encoder last

Update the way in which the virtual encoder requests the physical
encoder enables so that the master physical encoder is always
enabled last. This is done so that all of the slaves are fully
prepared when the master is enabled, and also allows the master
to update any necessary configuration during its enable callback.

Change-Id: Iff0d9cd08f8589889b50974fbc9819d96a15ab63
Signed-off-by: Clarence Ip <cip@codeaurora.org>