drm/i915: Simplify has_sagv function

drm/i915: Simplify has_sagv function

The specially case for SKL for not controlled sagv
is already taken care inside intel_enable_sagv, so there's
no need to duplicate the check here.

v2: Go one step further and remove skl special case. (Jani)
v3: Separate runtime status handle from has_sagv flag.
v4: Go back and accept simple Jani proposed solution.

Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181026200317.21726-1-rodrigo.vivi@intel.com