drm/vblank: Fix -Wformat compile warnings on some arches

drm/vblank: Fix -Wformat compile warnings on some arches

On some architectures like ppc64le and aarch64, compiling with
-Wformat=1 will throw the following warnings:

  In file included from drivers/gpu/drm/drm_vblank.c:33:
  drivers/gpu/drm/drm_vblank.c: In function 'drm_update_vblank_count':
  drivers/gpu/drm/drm_vblank.c:273:16: warning: format '%llu' expects
  argument of type 'long long unsigned int', but argument 4 has type
  'long int' [-Wformat=]
    DRM_DEBUG_VBL("updating vblank count on crtc %u:"
                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  ./include/drm/drm_print.h:407:22: note: in definition of macro
  'DRM_DEBUG_VBL'
    drm_dbg(DRM_UT_VBL, fmt, ##__VA_ARGS__)
                        ^~~
  drivers/gpu/drm/drm_vblank.c:274:22: note: format string is defined here
           " current=%llu, diff=%u, hw=%u hw_last=%u\n",
                     ~~~^
                     %lu

So, fix that with a typecast.

Co-developed-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20200521204647.2578479-1-lyude@redhat.com