drm/amd/display: Allocate scratch space for DMUB CW7

[Why]
The scratch space can be used to pass data between x86 and DMCUB.  DMCUB
will manage the actually mapping of CW7 internally, driver does not
program the window.

[How]
Allocate extra space within the DMUB service's framebuffer for this
scratch space and expose them from the service for use in DC.

Signed-off-by: Wyatt Wood <wyatt.wood@amd.com>
Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>