msm: mdss: Avoid deadlock in interrupt handler

Reduce the locking scope in irq_dispatch so that
mdss_lock is not held when the irq_handler is dispatched.
The dispatched irq_handler will try to acquire mdp_lock but
another active process can already have mdp_lock and tries to
procure mdss_lock creating a deadlock situation. Not locking
with mdss_lock is acceptable as the calls will be synchronized
through mdp_lock from MDP clients.

Change-Id: I586b7e00c046dd4081b4e838e62e0a0556f4c717
Signed-off-by: Sree Sesha Aravind Vadrevu <svadrevu@codeaurora.org>