drm/amd/display: Add analog link detection (v2)

drm/amd/display: Add analog link detection (v2)

Analog displays typically have a DDC connection which can be
used by the GPU to read EDID. This commit adds the capability
to probe analog displays using DDC, reading the EDID header and
deciding whether the analog link is connected based on the data
that was read.

Note that VGA has no HPD (hotplug detection), so we need to
to do analog link detection for VGA before checking HPD.

In case of DVI-I, while the connector supports HPD, not all
analog cables connect the HPD pins, so we can't rely on HPD
either.

For reference, see the legacy display code:
amdgpu_connector_vga_detect
amdgpu_display_ddc_probe

DAC load detection will be implemented in a separate commit.

v2:
Fix crash / black screen on newer GPUs during link detection.
Ignore HPD pin for analog connectors.

Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>