drm/amd/display: fix issue with eDP not detected on driver load

[Why]
HPD not going to be high if Panel VDD is off
And all AUX transaction will fail :(

[How]
1. Power on VDD before attempting detection if it isn't already on
2. Improve the robustness by having a retry mechanism on the
first DPCD read after VDD on. If a particular board always holds
HPD high incorrectly, the AUX access may fail, so we can retry
in those scenarios. This change would only improve logic
since it prevents AUX failure leading to bad resolution on internal
panel.
3. We should never need to re-detect internal panel, so logic
is re-arranged a bit to skip earlier.

Signed-off-by: Anthony Koo <Anthony.Koo@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>