drm/msm/dpu: Remove dpu_irq and unused functions

dpu_irq.c does some unneeded checks and passes control
to dpu_core_irq.c  The simple functions can be defined
in the same file where we use them and the files and
their associated hangers on can be deleted.

Additionally the postinstall hook isn't used even
in dpu_core_irq.c so zap that entire path.

v3: No changes

Reviewed-by: Sean Paul <sean@poorly.run>
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>