drm/xe/pm: Correct comment of xe_pm_set_vram_threshold()

drm/xe/pm: Correct comment of xe_pm_set_vram_threshold()

The parameter threshold is with size in MiB, not in bits.
Correct it to avoid any confusion.

v2: s/mb/MiB, s/vram/VRAM, fix return section. (Michal)

Fixes: 30c399529f4c ("drm/xe: Document Xe PM component")
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Shuicheng Lin <shuicheng.lin@intel.com>
Link: https://lore.kernel.org/r/20250708021450.3602087-2-shuicheng.lin@intel.com
Reviewed-by: Stuart Summers <stuart.summers@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>