gma500: First stage of renaming the driver

Rename the driver from modesetting to gma500 on as many places possible.
The driver now compiles as gma500_drv and can be selected in xorg config
as gma500. More work is needed but now modesetting and gma500 can live
side by side.

Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
gma500: Remove the README

We'll write a proper one later.

Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>