Documentation/gpu: drm-mm.rst: fix formatting for struct vm_operations_struct

drm-mm.rst contains some unformatted dump of the vm_operations_struct
structure. Add some C formatting around it and some context for the
dump. While there, update the structure to resemble the new signature
for the fault handler after commit 25d3db7600b8 (mm, fs: reduce fault,
page_mkwrite, and pfn_mkwrite to take only vmf).

Signed-off-by: Liviu Dudau <liviu.dudau@arm.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170131174109.13690-1-Liviu.Dudau@arm.com