drm/bridge: dw-mipi-dsi: fix dw_mipi_dsi_debugfs_show/write warnings

This fixes the following warnings while building in W=1 :
dw-mipi-dsi.c:1002:5: warning: no previous prototype for 'dw_mipi_dsi_debugfs_write' [-Wmissing-prototypes]
dw-mipi-dsi.c:1027:5: warning: no previous prototype for 'dw_mipi_dsi_debugfs_show' [-Wmissing-prototypes]

Fixes: e2435d69 ("drm/bridge: dw-mipi-dsi.c: Add VPG runtime config through debugfs")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Angelo Ribeiro <angelo.ribeiro@synopsys.com>
Cc: Maxime Ripard <maxime@cerno.tech>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20200907102711.23748-1-narmstrong@baylibre.com