drm/bridge: dw-mipi-dsi.c: remove unused header file

dw-mipi-dsi does not use any definition from drm_probe_helper.

Coverity output:
Event unnecessary_header:
Including .../include/drm/drm_probe_helper.h does not provide any
needed symbols.

Reviewed-by: Yannick Fertre <yannick.fertre@st.com>
Cc: Gustavo Pimentel <gustavo.pimentel@synopsys.com>
Cc: Joao Pinto <jpinto@synopsys.com>
Cc: Jose Abreu <jose.abreu@synopsys.com>
Signed-off-by: Angelo Ribeiro <angelo.ribeiro@synopsys.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/171ff1fb3918664a570dc8f2f34b446612505f76.1585832665.git.angelo.ribeiro@synopsys.com