drm/msm: kexec fixes

In kexec environment, we are more likely to encounter irq's already
enabled from previous environment.  At which point we find that writes
to disable/clear pending irq's are slightly less than useless without
first enabling clocks.

TODO: full blown state read-in so kexec'd kernel can inherit the mode
already setup.

Signed-off-by: Rob Clark <robdclark@gmail.com>