drm/panfrost: Add missing GPU feature registers

Three feature registers were declared but never actually read from the
GPU. Add THREAD_MAX_THREADS, THREAD_MAX_WORKGROUP_SIZE and
THREAD_MAX_BARRIER_SIZE so that the complete set are available.

Fixes: 4bced8be

 ("drm/panfrost: Export all GPU feature registers")
Signed-off-by: Steven Price <steven.price@arm.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20191014151515.13839-1-steven.price@arm.com