drm/vmwgfx: Don't double-free the mode stored in par->set_mode

When calling vmw_fb_set_par(), the mode stored in par->set_mode gets free'd
twice. The first free is in vmw_fb_kms_detach(), the second is near the
end of vmw_fb_set_par() under the name of 'old_mode'. The mode-setting code
only works correctly if the mode doesn't actually change. Removing
'old_mode' in favor of using par->set_mode directly fixes the problem.

Cc: <stable@vger.kernel.org>
Fixes: a278724a

 ("drm/vmwgfx: Implement fbdev on kms v2")
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Deepak Rawat <drawat@vmware.com>
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>