drm/radeon/dpm: add late_enable for rs780/rs880/rv6xx

Make sure interrupts are enabled before we enable
thermal interrupts.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

diff --cc drivers/gpu/drm/radeon/r600_dpm.c
Simple merge

diff --cc drivers/gpu/drm/radeon/radeon_asic.c
Simple merge

diff --cc drivers/gpu/drm/radeon/radeon_asic.h
Simple merge