drivers display_nrf_led_matrix: Init variable to avoid compile warning

Initialize row_index to avoid a compile warning.
The warning seems incorrect, as row_index is initialized in the first
interation of the loop, but the overhead of initializing it is trivial,
so we may as well just do it.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>