Start using device specific parameters instead of module parameters for
most things. The module parameters become the immutable initial values
for i915 parameters. The device specific parameters in i915->params
start life as a copy of i915_modparams. Any later changes are only
reflected in the debugfs.

The stragglers are:

* i915.force_probe and i915.modeset. Needed before dev_priv is
  available. This is fine because the parameters are read-only and never
  modified.

* i915.verbose_state_checks. Passing dev_priv to I915_STATE_WARN and
  I915_STATE_WARN_ON would result in massive and ugly churn. This is
  handled by not exposing the parameter via debugfs, and leaving the
  parameter writable in sysfs. This may be fixed up in follow-up work.

* i915.inject_probe_failure. Only makes sense in terms of the module,
  not the device. This is handled by not exposing the parameter via
  debugfs.

v2: Fix uc i915 lookup code (Michał Winiarski)

Cc: Juha-Pekka Heikkilä <juha-pekka.heikkila@intel.com>
Cc: Venkata Sandeep Dhanalakota <venkata.s.dhanalakota@intel.com>
Cc: Michał Winiarski <michal.winiarski@intel.com>
Reviewed-by: NRodrigo Vivi <rodrigo.vivi@intel.com>
Acked-by: NMichał Winiarski <michal.winiarski@intel.com>
Signed-off-by: NJani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20200618150402.14022-1-jani.nikula@intel.com