i915: Fix wpos_tex vs. -1 comparison

i915: Fix wpos_tex vs. -1 comparison

wpos_tex used to be a GLuint so assigning -1 to it and
later comparing with -1 worked correctly, but commit
c349031c27b7 ("i915: Fix texcoord vs. varying collision in
fragment programs") changed wpos_tex to uint8_t and hence
broke the comparison. To fix this define a more explicit
invalid value for wpos_tex.

gcc warns us:
i915_fragprog.c:1255:57: warning: comparison is always true due to limited range of data type [-Wtype-limits]
    if (inputsRead & VARYING_BITS_TEX_ANY || p->wpos_tex != -1) {
                                                         ^

And clang says:
i915_fragprog.c:1255:57: warning: comparison of constant -1 with expression of type 'uint8_t' (aka 'unsigned char') is always true [-Wtautological-constant-out-of-range-compare]
   if (inputsRead & VARYING_BITS_TEX_ANY || p->wpos_tex != -1) {
                                            ~~~~~~~~~~~ ^  ~~

Cc: Chih-Wei Huang <cwhuang@android-x86.org>
Cc: Eric Anholt <eric@anholt.net>
Cc: Ian Romanick <ian.d.romanick@intel.com>
Cc: mesa-stable@lists.freedesktop.org
Fixes: c349031c27b7 ("i915: Fix texcoord vs. varying collision in fragment programs")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>