staging: fbtft: use init function instead of init sequence

This patch converts the default init sequence of the ST7789V
display controller into an init function, as init sequences
are considered deprecated by the maintainers of fbtft.

Signed-off-by: Dennis Menschel <menschel-d@posteo.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>