drm/amd/display: Fix DIG_HPD_SELECT for USB4 display endpoints.

[Why]
DIB_BE_CNTL<i>.DIG_HPD_SELECT selects the HPD block being used
by the display endpoint assigned to DIG<i>. In the case of USB4
display endpoints, no physical HPD block is assigned.

[How]
Setting DIB_BE_CNTL<i>.DIG_HPD_SELECT to 5 indicates that no HPD
is assigned to a display endpoint. Firmware decrements the
HPD_SELECT value by 1 before writing it to the register.

Reviewed-by: Meenakshikumar Somasundaram <meenakshikumar.somasundaram@amd.com>
Acked-by: Wayne Lin <Wayne.Lin@amd.com>
Acked-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Jimmy Kizito <Jimmy.Kizito@amd.com>