drm/amdgpu/discovery: stop taking psp header into account

psp will write a header to vram, but the value exposed in
RCC_CONFIG_MEMSIZE does not include the memory that this header is
written to. Therefore, the interpretation of the table does not need to
take the psp header into account.

Signed-off-by: Xiaojie Yuan <xiaojie.yuan@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>