drm/dbi: Fix SPI Type 1 (9-bit) transfer

The function mipi_dbi_spi1_transfer() will transfer its payload as 9-bit
data, the 9th (MSB) bit being the data/command bit. In order to do that,
it unpacks the 8-bit values into 16-bit values, then sets the 9th bit if
the byte corresponds to data, clears it otherwise. The 7 MSB are
padding. The array of now 16-bit values is then passed to the SPI core
for transfer.

This function was broken since its introduction, as the length of the
SPI transfer was set to the payload size before its conversion, but the
payload doubled in size due to the 8-bit -> 16-bit conversion.

Fixes: 02dd95fe3169 ("drm/tinydrm: Add MIPI DBI support")
Cc: <[email protected]> # 5.4+
Signed-off-by: Paul Cercueil <[email protected]>
Reviewed-by: Sam Ravnborg <[email protected]>
Reviewed-by: Noralf Trønnes <[email protected]>
Signed-off-by: Sam Ravnborg <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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