drm/i915: Drop useless HAS_PSR() check

It is safe to call intel_psr_disable() on a platform without PSR. We
don't have such a check when calling intel_psr_enable() either.

v2: Don't drop the HAS_DDI check quite yet (Rodrigo)

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170920151236.5864-1-ville.syrjala@linux.intel.com