drm/bridge: dw-mipi-dsi.c: Add VPG runtime config through debugfs

Add support for the video pattern generator (VPG) BER pattern mode and
configuration in runtime.

This enables using the debugfs interface to manipulate the VPG after
the pipeline is set.
Also, enables the usage of the VPG BER pattern.

Changes in v2:
  - Added VID_MODE_VPG_MODE
  - Solved incompatible return type on __get and __set

Reported-by: kbuild test robot <lkp@intel.com>
Reported-by: Adrian Pop <pop.adrian61@gmail.com>
Signed-off-by: Angelo Ribeiro <angelo.ribeiro@synopsys.com>
Tested-by: Yannick Fertre <yannick.fertre@st.com>
Tested-by: Adrian Pop <pop.adrian61@gmail.com>
Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Cc: Gustavo Pimentel <gustavo.pimentel@synopsys.com>
Cc: Joao Pinto <jpinto@synopsys.com>
Cc: Jose Abreu <jose.abreu@synopsys.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/a809feb7d7153a92e323416f744f1565e995da01.1586180592.git.angelo.ribeiro@synopsys.com