panfrost: Remove unused anonymous enum variables.

panfrost: Remove unused anonymous enum variables.

This patch fix these build errors with GCC 10.

/usr/bin/ld: src/gallium/drivers/panfrost/libpanfrost.a(pan_resource.c.o):src/panfrost/midgard/midgard_compile.h:52: multiple definition of `pan_sysval'; src/gallium/drivers/panfrost/libpanfrost.a(pan_screen.c.o):src/panfrost/midgard/midgard_compile.h:52: first defined here
/usr/bin/ld: src/gallium/drivers/panfrost/libpanfrost.a(pan_resource.c.o):src/panfrost/midgard/midgard_compile.h:68: multiple definition of `pan_special_attributes'; src/gallium/drivers/panfrost/libpanfrost.a(pan_screen.c.o):src/panfrost/midgard/midgard_compile.h:68: first defined here

Fixes: 7e8de5a707f7 ("panfrost: Implement system values")
Fixes: 306800d747bc ("pan/midgard: Lower gl_VertexID/gl_InstanceID to attributes")
Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3752>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3752>