drm/xe: Waste fewer instructions in emit_wa_job()

drm/xe: Waste fewer instructions in emit_wa_job()

I was debugging some unrelated issue and noticed the current code was
very verbose. We can improve it easily by using the more common batch
buffer building pattern.

Before:
                bb->cs[bb->len++] = MI_LOAD_REGISTER_REG | MI_LRR_DST_CS_MMIO;
     c4d:       41 8b 56 10             mov    0x10(%r14),%edx
     c51:       49 8b 4e 08             mov    0x8(%r14),%rcx
     c55:       8d 72 01                lea    0x1(%rdx),%esi
     c58:       41 89 76 10             mov    %esi,0x10(%r14)
     c5c:       c7 04 91 01 00 08 15    movl   $0x15080001,(%rcx,%rdx,4)
                        bb->cs[bb->len++] = entry->reg.addr;
     c63:       8b 08                   mov    (%rax),%ecx
     c65:       41 8b 56 10             mov    0x10(%r14),%edx
     c69:       49 8b 76 08             mov    0x8(%r14),%rsi
     c6d:       81 e1 ff ff 3f 00       and    $0x3fffff,%ecx
     c73:       8d 7a 01                lea    0x1(%rdx),%edi
     c76:       41 89 7e 10             mov    %edi,0x10(%r14)
     c7a:       89 0c 96                mov    %ecx,(%rsi,%rdx,4)
 ..etc..

After:
                *cs++ = MI_LOAD_REGISTER_REG | MI_LRR_DST_CS_MMIO;
     c52:       41 c7 04 24 01 00 08    movl   $0x15080001,(%r12)
     c59:       15
                        *cs++ = entry->reg.addr;
     c5a:       8b 10                   mov    (%rax),%edx
 ..etc..

Resulting in the following binary change:

	add/remove: 0/0 grow/shrink: 0/2 up/down: 0/-348 (-348)
	Function                                     old     new   delta
	xe_gt_record_default_lrcs.cold               304     296      -8
	xe_gt_record_default_lrcs                   2200    1860    -340
	Total: Before=13554, After=13206, chg -2.57%

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://lore.kernel.org/r/20250710-lrc-refactors-v2-7-a5e2ca03f6bd@intel.com
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>