drivers: display: st7796s: respect rgb_is_inverted in 8080 8 bit mode

Update ST7796s display driver to respect the setting of rgb_is_inverted
in 8080 8 bit mode, as it was previously not applied for this mode.

Also, simplify the logic check for 16 bit mode- the new check is
functionally equivalent, but no longer inverts both boolean values.

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