drm/i915: Record the default hw state after reset upon load

drm/i915: Record the default hw state after reset upon load

BugLink: https://bugs.launchpad.net/bugs/1862840
Take a copy of the HW state after a reset upon module loading by
executing a context switch from a blank context to the kernel context,
thus saving the default hw state over the blank context image.
We can then use the default hw state to initialise any future context,
ensuring that each starts with the default view of hw state.

v2: Unmap our default state from the GTT after stealing it from the
context. This should stop us from accidentally overwriting it via the
GTT (and frees up some precious GTT space).

Testcase: igt/gem_ctx_isolation
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171110142634.10551-7-chris@chris-wilson.co.uk
CVE-2020-8832

(backported from commit d2b4b97933f5adacfba42dc3b9200d0e21fbe2c4)
[tyhicks: Backport to 4.15:
 - The HAS_LOGICAL_RING_PREEMPTION() macro does not exist because we
   don't have commit a4598d17551a ("drm/i915: Rename helpers used for
   unwinding, use macro for can_preempt")]
Signed-off-by: Tyler Hicks <tyhicks@canonical.com>
Acked-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Acked-by: Timo Aaltonen <timo.aaltonen@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>