drm/amd/display: remove legacy fields of dc_plane_cap struct

The fields blends_with_above and blends_with_below of struct
dc_plane_cap (defined in dc/dc.h) are boolean and set to true by
default. All instances of a dc_plane_cap maintain the default values of
both. Also, there is only one if statement that checks those fields and
there would be the same effect if it was deleted (assuming that those
fields are always going to be true).

For this reason, considering both fields as legacy ones, this commit
removes them and the aforementioned if statement.

Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: David Tadokoro <davidbtadokoro@usp.br>
Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>