drivers: mipi_dbi_spi: splitting SPI write function

The more complex the SPI transfer algorithms become, the more
confusing the current implementation of the SPI write function
becomes. Furthermore, if further as yet unknown MIPI DBI modes
are to be supported, the scope of this implementation would
increase dramatically. With the splitting now introduced, the
existing SPI transfer algorithms are moved to individual
auxiliary functions and the SPI write function only focus on
the decision of the respective MIPI DBI mode and the device
lock/unlock.

Signed-off-by: Stephan Linz <linz@li-pro.net>