drm/i915: Check if the stolen memory "reserved" area is enabled or not

drm/i915: Check if the stolen memory "reserved" area is enabled or not

Apparently there are some machines that put semi-sensible looking values
into the stolen "reserved" base and size, except those values are actually
outside the stolen memory. There is a bit in the register which
supposedly could tell us whether the reserved area is even enabled or
not. Let's check for that before we go trusting the base and size.

Cc: Tomi Sarvela <tomi.p.sarvela@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171102151737.23336-1-ville.syrjala@linux.intel.com
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>