drm/msm/dsi-staging: modify dsi interrupt handling

In the current implementation dsi driver enables DSI
interrupts on need basis, i.e. if some dsi operation
needs to be done then enables the irq state and once
the operation is finished disables the irq state.
This will cause issues when dsi driver is idle and HW
triggers any interrupt on master line. Since DSI irq
state is in disabled state, generic interrupt handler
of kernel will not call the corresponding ISR routine
and as a result interrupt will never get cleared and
will cause interrupt storm. Fix this by keeping DSI
irq state enabled as long as dsi core clocks are enabled
and once core clocks are disabled, dsi driver will
disable the irq state also.

Change-Id: I753798c0280000acdc497e295938264aae13111f
Signed-off-by: Sandeep Panda <spanda@codeaurora.org>