drm/msm/sde: refactor encoder irq code into common helpers

Reduce code duplication by refactoring common irq registration
and waiting functionality into utility functions in the sde
encoder that physical encoders can leverage.

Change-Id: I361d6e84238ae51161b57821ea4bf48a45c36942
Signed-off-by: Lloyd Atkinson <latkinso@codeaurora.org>