doc: drm: Update references to previously renamed files

Update references to reservation.c and reservation.h since these files
have been renamed to dma-resv.c and dma-resv.h respectively.

Cc: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/323401/?series=65037&rev=1
Signed-off-by: Anna Karas <anna.karas@intel.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190927111504.20136-1-anna.karas@intel.com