drm/core: add ioctl to query device/driver capabilities

We're coming to see a need to have a set of generic capability checks in
the core DRM, in addition to the driver-specific ioctls that already
exist.

This patch defines an ioctl to do as such, but does not yet define any
capabilities.

[airlied: drop the driver callback for now.]

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>