drm/i915/cmdparser: Add support for backward jumps

To keep things manageable, the pre-gen9 cmdparser does not
attempt to track any form of nested BB_START's. This did not
prevent usermode from using nested starts, or even chained
batches because the cmdparser is not strictly enforced pre gen9.

Instead, the existence of a nested BB_START would cause the batch
to be emitted in insecure mode, and any privileged capabilities
would not be available.

For Gen9, the cmdparser becomes mandatory (for BCS at least), and
so not providing any form of nested BB_START support becomes
overly restrictive. Any such batch will simply not run.

We make heavy use of backward jumps in igt, and it is much easier
to add support for this restricted subset of nested jumps, than to
rewrite the whole of our test suite to avoid them.

Add the required logic to support limited backward jumps, to
instructions that have already been validated by the parser.

Note that it's not sufficient to simply approve any BB_START
that jumps backwards in the buffer because this would allow an
attacker to embed a rogue instruction sequence within the
operand words of a harmless instruction (say LRI) and jump to
that.

We introduce a bit array to track every instr offset successfully
validated, and test the target of BB_START against this. If the
target offset hits, it is re-written to the same offset in the
shadow buffer and the BB_START cmd is allowed.

Note: This patch deliberately ignores checkpatch issues in the
cmdtables, in order to match the style of the surrounding code.
We'll correct the entire file in one go in a later patch.

v2: set dispatch secure late (Mika)
v3: rebase (Mika)
v4: Clear whitelist on each parse
    Minor review updates (Chris)
v5: Correct backward jump batching
v6: fix compilation error due to struct eb shuffle (Mika)

Cc: Tony Luck <tony.luck@intel.com>
Cc: Dave Airlie <airlied@redhat.com>
Cc: Takashi Iwai <tiwai@suse.de>
Cc: Tyler Hicks <tyhicks@canonical.com>
Signed-off-by: Jon Bloomfield <jon.bloomfield@intel.com>
Signed-off-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Chris Wilson <chris.p.wilson@intel.com>