i965: account for NIR uniforms without name

i965: account for NIR uniforms without name

Right now, the BRW linker code assumes nir_variable::name is always
non-NULL, but thanks to ARB_gl_spirv we will soon be linking SPIR-V
programs, and those explicitly require matching uniforms by location.
The name is just a debug hint.

Instead of checking for the name this patch makes it check for
var->num_state_slots on the assumption that everything that had an
internal name also had some state slots. This seems likely because the
two code paths that are taken when the name begins with "gl_" already
have an assert that var->state_slots is not NULL.

v2: simplified, most of it moved to glsl/nir/spirv (Neil Roberts)
v3: check for num_state_slots instead of the name. This is needed
    because we do actually have nameless builtins with SPIR-V such as
    PatchVerticesIn and we want them to hit the
    _mesa_add_state_reference code path (Neil Roberts)

Signed-off-by: Eduardo Lima <elima@igalia.com>
Signed-off-by: Neil Roberts <nroberts@igalia.com>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>