mesa: Fix clamping to -1.0 in snorm_to_float

mesa: Fix clamping to -1.0 in snorm_to_float

This patch fixes the return of a wrong value when x is lower than
-MAX_INT(src_bits) as the result would not be between [-1.0 1.0].

v2 by Samuel Iglesias <siglesias@igalia.com>:
    - Modify snorm_to_float() to avoid doing the division when
      x == -MAX_INT(src_bits)

Cc: 10.4 <mesa-stable@lists.freedesktop.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>