drm/amdgpu: drop the drm irq pre/post/un install callbacks

The preinstall callback didn't do anything because not all
of the IPs were initialized when it was called.

Move the postinstall setup into sequence in the driver.

The uninstall callback disabled all interrupt source, but
it got called too late in the driver sequence and caused problems
with IPs who already freed the relevant data structures.  Move
the call into the right place in the driver sequence.

Acked-by: Christian König <christian.koenig@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Tested-By: Mikita Lipski <mikita.lipski@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>