drm/msm: Make irq_postinstall optional

Allow the KMS operation 'irq_postinstall' to be optional
so that the target display drivers don't need to define
a dummy function if they don't need one.

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>