HACK: drm/tidss: dt property to force 16bit VP output to a 24bit bridge

On some boards the HDMI bridge takes a 24bit DPI input,  but only 16
data pins are actually enabled from the SoC.  This new property
forces the output to be RGB565 on a specific video port if the
bridge requests a 24bit RGB color space.

This assumes that the video port is connected like so:

SoC : Bridge
R0 ->   R3
R1 ->   R4
R2 ->   R5
R3 ->   R6
R4 ->   R7
G0 ->   G2
G1 ->   G3
G2 ->   G4
G3 ->   G5
G4 ->   G6
G5 ->   G7
B0 ->   B3
B1 ->   B4
B2 ->   B5
B3 ->   B6
B4 ->   B7

On the bridge side R0->R2, G0->G1, B0->B2 would be tied to ground.

The bridge sees 24bits of data,  but the lsb's are always zero.

Signed-off-by: Darren Etheridge <detheridge@ti.com>
Signed-off-by: Nishanth Menon <nm@ti.com>