msm: mdss: do not clear dsi-0 irq from dsi-1 with broadcast mode

At current broadcast mode implementation, dsi-1 try to clear dsi-0
irq at its interrupt service routine. This is not necessary.
When broadcast enabled, dsi-0 fetch command and wait for dsi-1 to
trigger cmd engine so that both dsi-0 and dsi-1 dcs commands are sent to
destination synchronously. Both controller receive CMD_DONE isr to
indicate dcs commands had been sent to destination. Therefore both
dsi-0 and dsi-1 should will clear its irq.

CRs-Fixed: 651042
Change-Id: Idd2ff593f8ab8b514bf760090b98aaaf4dd4429a
Signed-off-by: Kuogee Hsieh <khsieh@codeaurora.org>