brw: increase brw_reg::subnr size to 6 bits

Since Xe2, the registers are bigger and even the instruction
structures got updated to have 6 bits.

The way I detected this issue was when I tried to use
src/intel/executor to add the following instruction:

    add(8)          g6.8<1>UD      g4<8,8,1>UD    0x00000008UD    { align1 WE_all 1Q I@1 };

Executor would read this and end up emitting an add with dst being
g6<1>UD instead of what we wanted. It turns out that inside
brw_gram.y, at dstoperand and dstoperandex we do:

    $$.subnr = $$.subnr * brw_type_size_bytes($4);

which would overflow subnr back to 0.

The overflow doesn't seem to be a problem with code we emit directly
(unlike the code we parse, like above) due to the fact that we seem to
treat Xe2 registers as smaller all the way until we call phys_nr() and
phys_subnr() during code generation. The phys_subnr() function can
generate a value that would overflow reg.subnr, but this value is
never written back to reg.subnr, it's just returned as an unsigned
int.

Fixes: e9f63df2f2 ("intel/dev: Enable LNL PCI IDs without INTEL_FORCE_PROBE")
Reviewed-by: Caio Oliveira <caio.oliveira@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/33539>

src/intel/compiler/brw_reg.h

@@ -162,8 +162,8 @@ typedef struct brw_reg {
          unsigned negate:1;             /* source only */
          unsigned abs:1;                /* source only */
          unsigned address_mode:1;       /* relative addressing, hopefully! */
-         unsigned pad0:16;
-         unsigned subnr:5;              /* :1 in align16 */
+         unsigned pad0:15;
+         unsigned subnr:6;              /* :1 in align16 */
       };
       uint32_t bits;
    };