i965: Call spirv_to_nir() instead of glsl_to_nir() for SPIR-V shaders

i965: Call spirv_to_nir() instead of glsl_to_nir() for SPIR-V shaders

This is the main fork of the shader compilation code-path, where a NIR
shader is obtained by calling spirv_to_nir() or glsl_to_nir(),
depending on its nature..

v2: Use 'spirv_data' member from gl_linked_shader to know which method
   to call. (Timothy Arceri)

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>