drm/radeon: don't touch DCE or VGA regs on Hainan (v3)

Hainan has no display hardware:
- no DCE (crtc, uniphy, dac, etc.)
- no VGA

v2: fix bios fetch
v3: fix interrupts

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>