pan/bifrost: Manually constant fold register class

pan/bifrost: Manually constant fold register class

Fixes errors for some people building Mesa:

../src/panfrost/bifrost/bifrost_sched.c:32:31: error: initializer
element is not constant
 const unsigned max_vec2_reg = max_primary_reg / 2;

../src/panfrost/bifrost/bifrost_sched.c:33:31: error: initializer
element is not constant
 const unsigned max_vec3_reg = max_primary_reg / 4; // XXX: Do we need
to align vec3 to vec4 boundary?

../src/panfrost/bifrost/bifrost_sched.c:34:31: error: initializer
element is not constant
 const unsigned max_vec4_reg = max_primary_reg / 4;

../src/panfrost/bifrost/bifrost_sched.c:35:32: error: initializer
element is not constant
 const unsigned max_registers = max_primary_reg +

../src/panfrost/bifrost/bifrost_sched.c:40:28: error: initializer
element is not constant
 const unsigned vec2_base = primary_base + max_primary_reg;

../src/panfrost/bifrost/bifrost_sched.c:41:28: error: initializer
element is not constant
 const unsigned vec3_base = vec2_base + max_vec2_reg;

../src/panfrost/bifrost/bifrost_sched.c:42:28: error: initializer
element is not constant
 const unsigned vec4_base = vec3_base + max_vec3_reg;

../src/panfrost/bifrost/bifrost_sched.c:43:27: error: initializer
element is not constant
 const unsigned vec4_end = vec4_base + max_vec4_reg;

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>