Use helper functions to simplify _DSM related code in i915 driver.

Function intel_dsm() is used to check functions supported by ACPI _DSM
method, but it has strange check for special value 0x80000002. After
digging into nouveau driver, I think the check is copied from nouveau
driver and is useless for i915 driver, so remove it.
Acked-by: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Jiang Liu <jiang.liu@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>