i915: Fix texcoord vs. varying collision in fragment programs

i915: Fix texcoord vs. varying collision in fragment programs

i915 fragment programs utilize the texture coordinate registers
for both texture coordinates and varyings. Unfortunately the
code doesn't check if the same index might be in use for both.
It just naively uses the index to pick a texture unit, which
could lead to collisions.

Add an extra mapping step to allocate non conflicting texture
units for both uses.

The issue can be reproduced with a pair of simple shaders like
these:
 attribute vec4 in_mod;
 varying vec4 mod;
 void main() {
   mod = in_mod;
   gl_TexCoord[0] = gl_MultiTexCoord0;
   gl_Position = gl_ModelViewProjectionMatrix * gl_Vertex;
 }

 varying vec4 mod;
 uniform sampler2D tex;
 void main() {
   gl_FragColor = texture2D(tex, vec2(gl_TexCoord[0])) * mod;
 }

Fixes many piglit tests on i915:

    glsl-link-varyings-2
    glsl-orangebook-ch06-bump
    interpolation-none-gl_frontcolor-smooth-fixed
    interpolation-none-gl_frontcolor-smooth-none
    interpolation-none-gl_frontcolor-smooth-vertex
    interpolation-none-gl_frontsecondarycolor-smooth-fixed
    interpolation-none-gl_frontsecondarycolor-smooth-vertex
    interpolation-none-gl_frontsecondarycolor-smooth-none
    interpolation-none-other-flat-fixed
    interpolation-none-other-flat-none
    interpolation-none-other-flat-vertex
    interpolation-none-other-smooth-fixed
    interpolation-none-other-smooth-none
    interpolation-none-other-smooth-vertex

v2 [idr]: Minor formatting tweaks.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Cc: "11.0" <mesa-stable@lists.freedesktop.org>
(cherry picked from commit c349031c27b7f66151f07d785625c585e10a92c2)