drm/msm/sde: Register IRQ for CWB only for valid HW index

Concurrent writeback is currently supported only from LM2-PP2
indexes. If lower index LM/PP is selected then valid IRQ is not
registered resulting in error logging though non-fatal. Fix
this by registering CWB IRQ only if its valid.

Change-Id: I83831fa610730d4bea3f3ad9b96732126b6b2127
Signed-off-by: Jayant Shekhar <jshekhar@codeaurora.org>