gpu/drm: fix a few kernel-doc "/**" mark warnings

The opening comment mark "/**" is reserved for kernel-doc comments, so
it will generate warnings for comments that are not kernel-doc with
"make W=1". For example,

drivers/gpu/drm/drm_memory.c:2: warning: Cannot understand  * \file
drm_memory.c
Signed-off-by: Qian Cai <cai@lca.pw>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/1563198173-7317-1-git-send-email-cai@lca.pw

drivers/gpu/drm/drm_agpsupport.c

-/**
+/*
  * \file drm_agpsupport.c
  * DRM support for AGP/GART backend
  *

drivers/gpu/drm/drm_dma.c

-/**
+/*
  * \file drm_dma.c
  * DMA IOCTL and function support
  *

drivers/gpu/drm/drm_legacy_misc.c

-/**
+/*
  * \file drm_legacy_misc.c
  * Misc legacy support functions.
  *

drivers/gpu/drm/drm_lock.c

-/**
+/*
  * \file drm_lock.c
  * IOCTLs for locking
  *

drivers/gpu/drm/drm_memory.c

-/**
+/*
  * \file drm_memory.c
  * Memory management wrappers for DRM
  *

drivers/gpu/drm/drm_scatter.c

-/**
+/*
  * \file drm_scatter.c
  * IOCTLs to manage scatter/gather memory
  *

drivers/gpu/drm/drm_vm.c

-/**
+/*
  * \file drm_vm.c
  * Memory mapping for DRM
  *