drivers: mipi_dbi_spi: splitting SPI read function

The more complex the SPI transfer algorithms become, the more
confusing the current implementation of the SPI command read
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 read 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>