drm/gma500/mrst: Replace WMs and chickenbits with values from EMGD

For the minnowboard to work the values found in EMGD are required.
This might break Oaktrail but without hardware to test with I cannot
really tell (and do not really care).

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