drm/xe: Fix documentation heading levels in xe_guc_pc.c

Sphinx reports htmldocs warnings:

  Documentation/gpu/xe/xe_firmware:31: ./drivers/gpu/drm/xe/xe_guc_pc.c:76: ERROR: A level 2 section cannot be used here.
  Documentation/gpu/xe/xe_firmware:31: ./drivers/gpu/drm/xe/xe_guc_pc.c:87: ERROR: A level 2 section cannot be used here.

The xe_guc_pc.c documentation is included inside xe_firmware.rst.
The headers in the C file currently use '=' underlines, which conflict with
the parent document's section levels.

Fix this by demoting "Frequency management" and "Render-C States" headers
from '=' to '-' to correctly nest them as subsections.

Build environment: Python 3.13.7 Sphinx 8.2.3 docutils 0.22.3

Signed-off-by: Swaraj Gaikwad <swarajgaikwad1925@gmail.com>
Link: https://patch.msgid.link/20251209094836.18589-1-swarajgaikwad1925@gmail.com
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>

Author: Swaraj-1925

drivers/gpu/drm/xe/xe_guc_pc.c

@@ -76,7 +76,7 @@
  * exposes a programming interface to the host for the control of SLPC.
  *
  * Frequency management:
- * =====================
+ * ---------------------
  *
  * Xe driver enables SLPC with all of its defaults features and frequency
  * selection, which varies per platform.
@@ -87,7 +87,7 @@
  * for any workload.
  *
  * Render-C States:
- * ================
+ * ----------------
  *
  * Render-C states is also a GuC PC feature that is now enabled in Xe for
  * all platforms.