i965: fixed clamping in set_scissor_bits when the y is flipped

i965: fixed clamping in set_scissor_bits when the y is flipped

Calculating the scissor rectangle fields with the y flipped (0 on top)
can generate negative values that will cause assertion failure later on
as the scissor fields are all unsigned. We must clamp the bbox values
again to make sure they don't exceed the fb_height. Also fixed a
calculation error.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=108999
          https://bugs.freedesktop.org/show_bug.cgi?id=109594

v2:
   - I initially clamped the values inside the if (Y is flipped) case
   and I made a mistake in the calculation: the clamp of the bbox[2] should
   be a check if (bbox[2] >= fbheight) bbox[2] = fbheight - 1 instead and I
   shouldn't have changed the ScissorRectangleYMax calculation. As the
   fixed code is equivalent with using CLAMP instead of MAX2 at the top of
   the function when bbox[2] and bbox[3] are calculated, and the 2nd is more
   clear, I replaced it. (Nanley Chery)

v3:
   - Reversed the CLAMP change in bbox[3] as the API guarantees that the
   viewport height is positive. (Nanley Chery)

v4:
  - Added nomination for the mesa-stable branch and the link to the second
  bugzilla bug (Nanley Chery)

CC: <mesa-stable@lists.freedesktop.org>
Tested-by: Paul Chelombitko <qamonstergl@gmail.com>
Reviewed-by: Nanley Chery <nanley.g.chery@intel.com>