mdss: Separate isr registration and interrupt enable/disable

MDSS has a single irq line for different hw in the sub-system where each
hw registers their isr with MDSS and MDSS will divert interrupt to
respective isr. But if any sub hw enables their irq before registering
respective isr then it could lead to interrupts without isr. This will
stall the CPU and result in watchdog bark. Provide safeguard to this
situation by separating isr registration and interrupt enable/disable
where isr registration is done during probe or system init time.

Change-Id: Ib1b25fd7eb508fda0d9b1b341fe809bff779ebf7
CRs-Fixed: 472072
Signed-off-by: Ujwal Patel <ujwalp@codeaurora.org>