drm/i915/gt: Create gt/gtN/.defaults/ for per gt sysfs defaults

Create a gt/gtN/.defaults/ directory (similar to
engine/<engine-name>/.defaults/) to expose default parameter values for
each gt in sysfs. This allows userspace to restore default parameter values
after they have changed. The empty 'struct gt_defaults' will be populated
by subsequent patches.

v2: Changed 'struct intel_rps_defaults rps_defaults' to
    'struct gt_defaults defaults' (Andi)

Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: Andi Shyti <andi.shyti@linux.intel.com>
Signed-off-by: Ashutosh Dixit <ashutosh.dixit@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/be7c30d0ae58be9d8d5b8242ba00a1b2825e63ad.1658192398.git.ashutosh.dixit@intel.com