It seems that the DMC likes to transition between the DC states a lot when
there are no connected displays (no active power domains) during command
submission.

This activity on DC states has a negative impact on the performance of the
chip with huge latencies observed in the interrupt handlers and elsewhere.
Simple tests like igt/gem_latency -n 0 are slowed down by a factor of
eight.

Work around it by introducing a new power domain named,
POWER_DOMAIN_GT_IRQ, associtated with the "DC off" power well, which is
held for the duration of command submission activity.

CNL has the same problem which will be addressed as a follow-up. Doing
that requires a fix for a DC6 context corruption problem in the CNL DMC
firmware which is yet to be released.

v2:
 * Add commit text as comment in i915_gem_mark_busy. (Chris Wilson)
 * Protect macro body with braces. (Jani Nikula)

v3:
 * Add dedicated power domain for clarity. (Chris, Imre)
 * Commit message and comment text updates.
 * Apply to all big-core GEN9 parts apart for Skylake which is pending DMC
   firmware release.

v4:
 * Power domain should be inner to device runtime pm. (Chris)
 * Simplify NEEDS_CSR_GT_PERF_WA macro. (Chris)
 * Handle async DMC loading by moving the GT_IRQ power domain logic into
   intel_runtime_pm. (Daniel, Chris)
 * Include small core GEN9 as well. (Imre)

v5
 * Special handling for async DMC load is not needed since on failure the
   power domain reference is kept permanently taken. (Imre)

v6:
 * Drop the NEEDS_CSR_GT_PERF_WA macro since all firmwares have now been
   deployed. (Imre, Chris)
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=100572
Testcase: igt/gem_exec_nop/headless
Cc: Imre Deak <imre.deak@intel.com>
Acked-by: Chris Wilson <chris@chris-wilson.co.uk> (v2)
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Dmitry Rogozhkin <dmitry.v.rogozhkin@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch> (v5)
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
[Imre: Add note about applying the WA on CNL as a follow-up]
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171205132854.26380-1-tvrtko.ursulin@linux.intel.com