drm/msm/dpu: enable master-slave encoders explicitly

Identify slave-master encoders during initialization and enable
the encoders explicitly as the current logic has redundant and
ambiguous loops.

changes in v4:
	- identify master/slave encoder while adding
	  adding physical encoders(Sean)
changes in v5:
	- get rid of temporary variable for phys enc(Sean)

Signed-off-by: Jeykumar Sankaran <jsanka@codeaurora.org>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>