[Why]
amdgpu_dm.c requires changes for SI chipsets init and irq handlers registration

[How]
SI support: load_dmcu_fw(), amdgpu_dm_initialize_drm_device(), dm_early_init()
Add DCE6 specific dce60_register_irq_handlers() function

(v1) NOTE: As per Kaveri and older amdgpu.dc=1 kernel cmdline is required

(v2) fix for bc011f9 ("drm/amdgpu: Change SI/CI gfx/sdma/smu init sequence")
     remove CHIP_HAINAN support since it does not have physical DCE6 module

(v3) fix vblank irq support for DCE6 using ad hoc dce60_register_irq_handlers()
     replicating for vblank irq the behavior of dce110_register_irq_handlers()
     as per commit b57de80a ("drm/amd/display: Register on VLBLANK ISR.")

(v4) updated due to following kernel 5.2 commit:
     b2fddb13 ("drm/amd/display: Drop underlay plane support")
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Mauro Rossi <issor.oruam@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>