mesa: Fix RGBBuffers for renderbuffers with sized internal formats

mesa: Fix RGBBuffers for renderbuffers with sized internal formats

For texture attachments, 'f' is texImg->_BaseFormat, but for
renderbuffer attachments, 'f' is att->Renderbuffer->InternalFormat.

InternalFormat may be something like GL_RGB8, which causes our
(f == GL_RGB) check to fail.  Switch to using a proper _BaseFormat,
which drops the size.

Fixes dEQP-GLES31.functional.draw_buffers_indexed.random.
max_required_draw_buffers.15 on iris when combined with a driver fix.

Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Timur Kristóf <timur.kristof@gmail.com>