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

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>