drm/i915/guc: Don't store runtime GuC log level in modparam

Currently we are using modparam as placeholder for GuC log level.
Stop doing this and keep runtime GuC level in intel_guc_log struct.

v2:
- rename functions intel_guc_log_level_[get|set] to
intel_guc_log_[get|set]_level (Michał Wajdeczko)
- remove GEM_BUG_ON from intel_guc_log_get_level() (Michał Wajdeczko)

Signed-off-by: Piotr Piórkowski <piotr.piorkowski@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Michał Winiarski <michal.winiarski@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180604141947.8299-1-piotr.piorkowski@intel.com