disp: msm: sde: increase max number of mixers to 4

Increase the maximum number of mixers per crtc to 4 to
support 4LM use case. This change also increases the number
of data path to 4 to support 4LM in continuous splash handoff.

Change-Id: I4655017dcb405fad69513bebb8fd7f848fc5873d
Signed-off-by: Amine Najahi <anajahi@codeaurora.org>