i915: Add support for gl_FragData[0] for output color.

We advertised ARB_draw_buffers, but either fell back to software when
using this output, or assertion failed.  Fixes glsl-fs-fragdata-1, and
failures in some webgl conformance tests.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=39024
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=34906

src/mesa/drivers/dri/i915/i915_fragprog.c

@@ -210,6 +210,7 @@ get_result_vector(struct i915_fragment_program *p,
    case PROGRAM_OUTPUT:
       switch (inst->DstReg.Index) {
       case FRAG_RESULT_COLOR:
+      case FRAG_RESULT_DATA0:
          return UREG(REG_TYPE_OC, 0);
       case FRAG_RESULT_DEPTH:
          p->depth_written = 1;