i965: initialize SPIR-V capabilities

i965: initialize SPIR-V capabilities

Needed for ARB_gl_spirv. Those are not the same that the Intel vulkan
driver. From the ARB_spirv_extensions spec:

   "3. If a new GL extension is added that includes SPIR-V support via
   a new SPIR-V extension does it's SPIR-V extension also get
   enumerated by the SPIR_V_EXTENSIONS_ARB query?.

   RESOLVED. Yes. It's good to include it for consistency. Any SPIR-V
   functionality supported beyond the SPIR-V version that is required
   for the GL API version should be enumerated."

So in addition to the core SPIR-V support, there is the possibility of
specific GL extensions enabling specific SPIR-V extensions (so
capabilities). That would mean that it is possible that OpenGL and
Vulkan not having the same capabilities supported, even for the same
driver. For this reason it is better to keep them separated.

As an example: at the time of this patch writing Intel vulkan driver
support multiview, but there isn't any OpenGL multiview GL extension
supported.

Note: we initialize SPIR-V capabilities at brwCreateContext instead of
the usual brw_initialize_context_constants because we want to do that
only if the extension is enabled.

v2:
   * Rebase update (SpirVCapabilities not a pointer anymore)
   * Fill spirv capabilities for OpenGL >= 3.3 (Ian Romanick)

v3:
   * Drop multiview support, as i965 doesn't support any multiview GL
     extension (Jason)
   * Fill spirv capabilities only if the extension is enabled (Jason)

v4: Capabilities are supported only on gen7+. Added comment and assert
    (Jason)