MagTag SSD1680 FPC-7519rev.b: use VCOM=0x20 for correct contrast

The FPC-7519rev.b panel (User ID byte 0xca) shows a dark gray background
with the default VCOM=0x28. Setting VCOM=0x20 (-1.5V) gives a light gray
background with solid black text.

Surgical fix: dedicated ssd1680_vcom20_display_start_sequence and
DISPLAY_SSD1680_COLSTART_8_VCOM20 type routed to case 0xca: only.
The 0x44 and 0x00 panels are untouched.