drm/stm: Fix bus_flags handling

The drm_display_mode_to_videomode() does not populate DISPLAY_FLAGS_DE_LOW
or DISPLAY_FLAGS_PIXDATA_NEGEDGE flags in struct videomode. Therefore, no
matter what polarity the next bridge or display might require, these flags
are never set, and thus the LTDC GCR_DEPOL and GCR_PCPOL bits are never set
and the LTDC behaves as if both DISPLAY_FLAGS_PIXDATA_POSEDGE and
DISPLAY_FLAGS_DE_HIGH were always set.

The fix for this problem is taken almost verbatim from MXSFB driver. In
case there is a bridge attached to the LTDC, the bridge might have extra
polarity requirements, so extract bus_flags from the bridge and use them
for LTDC configuration. Otherwise, extract bus_flags from the connector,
which is the display.

Fixes: b759012c5fa7 ("drm/stm: Add STM32 LTDC driver")
Signed-off-by: Marek Vasut <[email protected]>
Signed-off-by: Yannick Fertre <[email protected]>
Cc: Alexandre Torgue <[email protected]>
Cc: Antonio Borneo <[email protected]>
Cc: Benjamin Gaignard <[email protected]>
Cc: Maxime Coquelin <[email protected]>
Cc: Philippe Cornu <[email protected]>
Cc: Sam Ravnborg <[email protected]>
Cc: Vincent Abriou <[email protected]>
Cc: Yannick Fertre <[email protected]>
Cc: [email protected]
Cc: [email protected]
To: [email protected]
Tested-by: Yannick Fertre <[email protected]>
Signed-off-by: Philippe Cornu <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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