drm/amdgpu: register a vga_switcheroo client for MacBooks with apple-gmux

Commit 3840c5bc ("drm/amdgpu: disentangle runtime pm and
vga_switcheroo") made amdgpu only register a vga_switcheroo client for
GPU's with PX, however AMD GPUs in dual gpu Apple Macbooks do need to
register, but don't have PX. Instead of AMD's PX, they use apple-gmux.

Use apple_gmux_detect() to identify these gpus, and
pci_is_thunderbolt_attached() to ensure eGPUs connected to Dual GPU
Macbooks don't register with vga_switcheroo.

Fixes: 3840c5bc

 ("drm/amdgpu: disentangle runtime pm and vga_switcheroo")
Link: https://lore.kernel.org/amd-gfx/20230210044826.9834-10-orlandoch.dev@gmail.com/
Signed-off-by: Orlando Chamberlain <orlandoch.dev@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>