drm/vc4: Add a getparam ioctl for getting the V3D identity regs.

As I extend the driver to support different V3D revisions, userspace
needs to know what version it's targeting.  This is most easily
detected using the V3D identity registers.

v2: Make sure V3D is runtime PM on when reading the registers.
v3: Switch to a 64-bit param value (suggested by Rob Clark in review)

Signed-off-by: Eric Anholt <eric@anholt.net>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch> (v2)
Reviewed-by: Rob Clark <robdclark@gmail.com> (v3, over irc)