drm/amd/display: do not override CURSOR_REQ_MODE when SubVP is not enabled

[Why]
HUBP_UNBOUNDED_REQ_MODE and CURSOR_REQ_MODE are normally set together.
In hubp32_prepare_subvp_buffering() call, CURSOR_REQ_MODE is set based on
whether SubVP is enabled or not.  For non MPO case, both REQ_MODE
registers are set to 1.  But since SubVP is not enabled, then
CURSOR_REQ_MODE is set to 0, overriding the previous value.

[How]
Do not set CURSOR_REQ_MODE to 0 if SubVP is not enabled.  This
will allow CURSOR_REQ_MODE to stay as 1 in the non MPO case.
Add note to follow up and check case for single pipe MPO and
SubVP enabled as this would cause both REQ_MODE registers to be
set to 0 but SubVP enabled would override CURSOR_REQ_MODE to 1.

Signed-off-by: Samson Tam <Samson.Tam@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>