drivers: mipi_dbi: mipi_dbi_spi: do not take spinlock

Taking a spinlock will result in interrupts being blocked in the MIPI
DBI driver, which is not desired behavior while issuing SPI transfers,
since the driver may use interrupts to drive the transfer

Fixes #68815

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>