drm/xe: Normalize double-underscore include guards to single-underscore

drm/xe: Normalize double-underscore include guards to single-underscore

Six headers use __XE_<NAME>_H__ (double-underscore prefix and suffix)
as their include guards. Normalize them to the single-underscore
_XE_<NAME>_H_ convention used by the rest of the xe codebase.

Files fixed:
  - xe_eu_stall.h:     __XE_EU_STALL_H__     -> _XE_EU_STALL_H_
  - xe_nvm.h:          __XE_NVM_H__          -> _XE_NVM_H_
  - xe_pxp.h:          __XE_PXP_H__          -> _XE_PXP_H_
  - xe_pxp_debugfs.h:  __XE_PXP_DEBUGFS_H__ -> _XE_PXP_DEBUGFS_H_
  - xe_pxp_submit.h:   __XE_PXP_SUBMIT_H__  -> _XE_PXP_SUBMIT_H_
  - xe_pxp_types.h:    __XE_PXP_TYPES_H__   -> _XE_PXP_TYPES_H_

No functional change.

Suggested-by: Nitin Gote <nitin.r.gote@intel.com>
Assisted-by: GitHub Copilot:claude-opus-4.6
Reviewed-by: Nitin Gote <nitin.r.gote@intel.com>
Link: https://patch.msgid.link/20260317215732.2208976-12-shuicheng.lin@intel.com
Signed-off-by: Shuicheng Lin <shuicheng.lin@intel.com>