drm/i915/gt: Cleanup kasan warning for on-stack (unsigned long) casting

Kasan is gving a warning for passing a u32 parameter into find_first_bit
(casting to a unsigned long *, with appropriate length restrictions):

[   44.678262] BUG: KASAN: stack-out-of-bounds in find_first_bit+0x2e/0x50
[   44.678295] Read of size 8 at addr ffff888233f4fc30 by task core_hotunplug/474
[   44.678326]
[   44.678358] CPU: 0 PID: 474 Comm: core_hotunplug Not tainted 5.9.0+ #608
[   44.678465] Hardware name: BESSTAR (HK) LIMITED GN41/Default string, BIOS BLT-BI-MINIPC-F4G-EX3R110-GA65A-101-D 10/12/2018
[   44.678500] Call Trace:
[   44.678534]  dump_stack+0x84/0xba
[   44.678569]  print_address_description.constprop.0+0x21/0x220
[   44.678605]  ? kmsg_dump_rewind_nolock+0x5f/0x5f
[   44.678638]  ? _raw_spin_lock_irqsave+0x6d/0xb0
[   44.678669]  ? _raw_write_lock_irqsave+0xb0/0xb0
[   44.678702]  ? set_task_cpu+0x1e0/0x1e0
[   44.678733]  ? find_first_bit+0x2e/0x50
[   44.678763]  kasan_report.cold+0x20/0x42
[   44.678794]  ? find_first_bit+0x2e/0x50
[   44.678825]  __asan_load8+0x69/0x90
[   44.678856]  find_first_bit+0x2e/0x50
[   44.679027]  __caps_show.isra.0+0x9e/0x1f0 [i915]

Since we are only using the shorter type for our own convenience,
accommodate kasan and use unsigned long.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201013110845.16127-1-chris@chris-wilson.co.uk