gpu/doc: Convert to markdown harder

This snippet...

    * Lock VMA manager for extended lookups. Only *_locked() VMA function calls
    * are allowed while holding this lock. All other contexts are blocked from VMA
    * until the lock is released via drm_vma_offset_unlock_lookup().

...causes markdown-enabled kernel-doc to barf:

    debian/build/build-doc/Documentation/DocBook/gpu.aux.xml:3247: parser error : Opening and ending tag mismatch: emphasis line 3247 and function
       *<function><emphasis>locked</function> VMA function calls are allowed while
                                             ^
    /root/airlied/debian/build/build-doc/Documentation/DocBook/gpu.aux.xml:3249: parser error : Opening and ending tag mismatch: function line 3249 and emphasis
       released via <function>drm</emphasis>vma_offset_unlock_lookup</function>.
                                            ^
    unable to parse /root/airlied/debian/build/build-doc/Documentation/DocBook/gpu.aux.xml

A quick workaround is to replace *_locked() by X_locked().

Cc: Danilo Cesar Lemes de Paula <danilo.cesar@collabora.co.uk>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
[danvet: Just drop the X_ too, the usual style is _unlocked, except
that _ seems to be what annoys markdown.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>