drm/imagination: Implement context creation/destruction ioctls

Implement ioctls for the creation and destruction of contexts. Contexts are
used for job submission and each is associated with a particular job type.

Changes since v8:
- Fixed one error path in pvr_stream_process_1()
- Corrected license identifiers

Changes since v5:
- Fix context release in final error path in pvr_context_create()

Changes since v3:
- Use drm_dev_{enter,exit}

Co-developed-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Sarah Walker <sarah.walker@imgtec.com>
Signed-off-by: Donald Robson <donald.robson@imgtec.com>