drm: xilinx: dp_sub: Use static for internal variables

drm: xilinx: dp_sub: Use static for internal variables

Fixing the warning,
drivers/gpu/drm/xilinx/xilinx_drm_dp_sub.c:379:30: warning: symbol 'dp_sub_debugfs' was not declared. Should it be static?
drivers/gpu/drm/xilinx/xilinx_drm_dp_sub.c:540:38: warning: symbol 'dp_sub_debugfs_reqs' was not declared. Should it be static?
  CC      drivers/gpu/drm/xilinx/xilinx_drm_dp_sub.o

Signed-off-by: Hyun Kwon <hyun.kwon@xilinx.com>
CC: Tejas Upadhyay <tejasu@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>