drm/arm/hdlcd: Simplify IRQ install/uninstall

Since we no longer need to conform to the structure of the various DRM
IRQ callbacks, we can streamline the code by consolidating the piecemeal
functions and passing around our private data structure directly. We're
also a platform device so should never see IRQ_NOTCONNECTED either.

Furthermore we can also get rid of all the unnecesary read-modify-write
operations, since on install we know we cleared the whole interrupt mask
before enabling the debug IRQs, and thus on uninstall we're always
clearing everything as well.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Acked-by: Liviu Dudau <liviu.dudau@arm.com>
Signed-off-by: Liviu Dudau <liviu.dudau@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/65cf7818b23c1a8629dc851f1d058ecb8a14849e.1655309413.git.robin.murphy@arm.com