drm/amd: Drop special case for yellow carp without discovery

`amdgpu_gmc_get_vbios_allocations` has a special case for how to
bring up yellow carp when amdgpu discovery is turned off. As this ASIC
ships with discovery turned on, it's generally dead code and worse it
causes `adev->mman.keep_stolen_vga_memory` to not be initialized for
yellow carp.

Remove it.

Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>