drm/i915: Fix docbook header for __intel_runtime_pm_get_if_active()

Fix the

Documentation/gpu/i915:22: /drivers/gpu/drm/i915/intel_runtime_pm.c:423: WARNING: Inline strong start-string without end-string.

warning from the htmldocs build.

Fixes: 9d58aa46 ("drm/i915: Fix the GT fence revocation runtime PM logic")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Acked-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20210330150118.1105079-1-imre.deak@intel.com