i915: Disable vertex texturing, fix a couple of asserts
Vertex texturing is not a required feature of the GL2.1 or GLES2, and you really don't want to be doing SW VS access of the write-combined texture data. Also, avoids memory corruption in deqp:
Test case 'dEQP-GLES2.functional.texture.vertex.cube.filtering.linear_mipmap_nearest_linear_repeat'..
Mesa: User error: GL_INVALID_ENUM in glGetIntegerv(pname=GL_MAJOR_VERSION)
Fail (Image comparison failed)
Test case 'dEQP-GLES2.functional.fragment_ops.depth_stencil.stencil_depth_funcs.stencil_equal_depth_always'..
==559181== Invalid read of size 4
==559181== at 0x641E8D0: i915_drm_buffer_unmap (i915_drm_buffer.c:204)
==559181== by 0x64151EB: i915_cleanup_vertex_sampling (i915_state.c:449)
==559181== by 0x640AEA7: i915_draw_vbo (i915_context.c:134)
==559181== by 0x640AEA7: i915_draw_vbo (i915_context.c:55)
==559181== by 0x61367B1: cso_draw_vbo (cso_context.c:1524)
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