drm/amd/display: Translate cursor position by source rect

[Why]
Cursor is drawn as part of the framebuffer for a plane on AMD hardware.
The cursor position on the framebuffer does not change even if the
source rect viewport for the cursor does. This causes the cursor to be
clipped.

The following IGT tests fail as a result of this issue:

- kms_plane_cursor@pipe-*-viewport-size-*

[How]
Offset cursor position by plane source rect viewport. If the viewport
is unscaled then the cursor is now correctly positioned on any
plane - primary or overlay.

There is still a hardware limitation for dealing with the cursor size
being incorrectly scaled but that's not something we can address.

Add some documentation explaining some of this in the code while we're
at it.

Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Zhan Liu <Zhan.Liu@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>