drm/i915/selftests: Only query RAPL for integrated power measurements

RAPL provides an on-package power measurements which does not encompass
discrete graphics, so let's avoid using the igfx masurements when testing
dgfx. Later we will abstract the simple librapl interface over hwmon so
that we can verify basic power consumption scenarios.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210412090526.30547-3-matthew.auld@intel.com