drm/amd/display: dpia should avoid encoder used by dp2

drm/amd/display: dpia should avoid encoder used by dp2

[WHY]
In current HPO DP2 implementation, driver would enable/disable DIG
encoder when configuring HPO DP2. Therefore, usb4 dp tunnelling should
not use the DIG encoder if the corresponded phy is used by a HPO DP2
stream.

[HOW]
A DP2 stream is treated as a dig stream.

Reviewed-by: Meenakshikumar Somasundaram <meenakshikumar.somasundaram@amd.com>
Signed-off-by: Peichen Huang <PeiChen.Huang@amd.com>
Signed-off-by: Roman Li <roman.li@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>