drm: Switch to use DEVFREQ_GOV_SIMPLE_ONDEMAND constant

Since governor name is defined by DEVFREQ framework internally, use the
macro definition instead of using the name directly.

Signed-off-by: Yue Hu <huyue2@yulong.com>
Reviewed-by: Chanwoo Choi <cw00.choi@samsung.com>
Acked-by: Jordan Crouse <jcrouse@codeaurora.org> for the msm part.
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190725035239.1192-1-zbestahu@gmail.com