disp: msm: sde: decouple FETCH_PIPE_ACTIVE logic from setup_blendstages

Setup blendstages is done per LM but FETCH_PIPE_ACTIVE is per CTL.
Overloading mixer blendstage setup with fetch pipe logic can lead
to HW programming errors. Refactor the logic for setting
FETCH_PIPE_ACTIVE by adding a new op that allows caller to provide
a bitmask of all pipes required to be active on this CTL. This new
logic includes support for:
 - 4LM use-cases, staging pipes for all LMs within a CRTC
 - Demura fetch-pipe without need for tracking via active_cfg (removed)

Also, lower the cyclomatic complexity in setup_blendstages by moving
the logic for obtaining the mixer config settings in to a helper
function.

Change-Id: I2907b359ffad5734be5b06f44919b5ddb1ef3f7c
Signed-off-by: Steve Cohen <cohens@codeaurora.org>