drm/msm/dpu: define interrupt register names

In order to make mdss_irqs readable (and error-prone) define names for
interrupt register indices.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Abhinav Kumar <abhinavk@codeaurora.org>
Link: https://lore.kernel.org/r/20210516202910.2141079-4-dmitry.baryshkov@linaro.org


Signed-off-by: Rob Clark <robdclark@chromium.org>