drm/panthor: Allow driver compilation

Now that all blocks are available, we can add/update Kconfig/Makefile
files to allow compilation.

v4:
- Add Steve's R-b

v3:
- Add a dep on DRM_GPUVM
- Fix dependencies in Kconfig
- Expand help text to (hopefully) describe which GPUs are to be
  supported by this driver and which are for panfrost.

Co-developed-by: Steven Price <steven.price@arm.com>
Signed-off-by: Steven Price <steven.price@arm.com>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Acked-by: Steven Price <steven.price@arm.com> # MIT+GPL2 relicensing,Arm
Acked-by: Grant Likely <grant.likely@linaro.org> # MIT+GPL2 relicensing,Linaro
Acked-by: Boris Brezillon <boris.brezillon@collabora.com> # MIT+GPL2 relicensing,Collabora
Reviewed-by: Steven Price <steven.price@arm.com>