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>