drvers: display: ili9xxx: fix usage of MIPI buffer descriptor

Fix usage of MIPI buffer descriptor in ili9xxx driver. Previously, the
buffer descriptor size was being set to display buffer size. For cases
where the write height/width  was not equal to the size of the buffer, this
resulted in additional data being written that was not needed. To
resolve this, calculate the mipi descriptor buffer size in the driver

Also, remove the unconditional setting of mipi_desc.height, as this
would override the previous (correct) setting.

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