drm/panel: add panel driver for Xinpeng XPP055C272 panels

drm/panel: add panel driver for Xinpeng XPP055C272 panels

Base on the somewhat similar Rocktech driver but adapted for
panel-specific init of the XPP055C272.

changes in v5:
- drop error message when backlight not found, no other panel
  does that and if needed it should live in drm_panel_of_backlight
changes in v4:
  none
changes in v3:
- remove wrong negative sync flags from display-mode to fix a display
  artifact of the output getting move a tiny bit to the right
changes in v2:
- move to drm-panel-internal backlight handling (Sam)
- adapt to changes that happened to drm_panel structs+functions (Sam)
- sort includes (Sam)
- drop unnecessary DRV_NAME constant (Sam)
- do mipi_dsi_dcs_exit_sleep_mode and mipi_dsi_dcs_set_display_on
  in panel prepare (not init_sequence) to keep symmetric (Sam)

Signed-off-by: Heiko Stuebner <heiko.stuebner@theobroma-systems.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20191224112907.30758-3-heiko@sntech.de