Staging: fbtft: Fix too many leading tabs warning

This is a patch to the fb_agm1264k-fl.c file that fixes up the following
issue:

WARNING: Too many leading tabs - consider code refactoring

Add new function iterate_diffusion_matrix() to refactor a piece of code
inside write_vmem().

Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>