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: 30c3995 ("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>

Author: ShuichengLin

drivers/gpu/drm/xe/xe_pm.c

@@ -761,11 +761,13 @@ void xe_pm_assert_unbounded_bridge(struct xe_device *xe)
 }
 
 /**
- * xe_pm_set_vram_threshold - Set a vram threshold for allowing/blocking D3Cold
+ * xe_pm_set_vram_threshold - Set a VRAM threshold for allowing/blocking D3Cold
  * @xe: xe device instance
- * @threshold: VRAM size in bites for the D3cold threshold
+ * @threshold: VRAM size in MiB for the D3cold threshold
  *
- * Returns 0 for success, negative error code otherwise.
+ * Return:
+ * * 0		- success
+ * * -EINVAL	- invalid argument
  */
 int xe_pm_set_vram_threshold(struct xe_device *xe, u32 threshold)
 {