drm/i915: don't run hsw power well code on !hsw

Dumps annoying noise into the dmesg:

[drm:intel_set_power_well] *ERROR* Timeout enabling power well

Reported-by: Sedat Dilek <sedat.dilek@gmail.com>
Cc: Sedat Dilek <sedat.dilek@gmail.com>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Tested-by: Sedat Dilek <sedat.dilek@gmail.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>