i965: fix noop_scissor range issue on width/height

i965: fix noop_scissor range issue on width/height

If scissor X or Y was set to a negative value then the previous
code might have indicated noop scissors when the scissor range
actually was masking a portion of the framebuffer.

Since fb->_Xmin, _Xmax, _Ymin and _Ymax take scissors into
account, we can use these to test for a noop scissor.

Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Matt Turner <mattst88@gmail.com>
Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>