drm/i915: Reorder gen3/4 swizzle detection logic

drm/i915: Reorder gen3/4 swizzle detection logic

g33/i964g/g45 are the exceptional cases when it comes to
the swizzle detection. Let's reorder the code to handle
them first and let everything else be handled by the
else branch. This allows us to unset .is_mobile for the
desktop PNV variant (which supposedly must follow the
"mobile" path here).

Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190318165633.28924-1-ville.syrjala@linux.intel.com
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>