drivers: display: fix zero-buffers-in-SRAM case in DCNANO LCDIF

If CONFIG_MCUX_DCNANO_LCDIF_EXTERNAL_FB_MEM is not set and
CONFIG_MCUX_DCNANO_LCDIF_FB_NUM is zero, a division by zero occurs.
Use the same framebuffer size definition as in the external-framebuffer
case to resolve this.

Fixes #58908.

Signed-off-by: Aedan Cullen <aedan@aedancullen.com>