drm/sti: set mixer background color through module param

Add bkgcolor module parameter that allow to change the background
color of the mixer. It can be set with an RGB value coded as 0xRRGGBB.
The default value is black.

Signed-off-by: Vincent Abriou <vincent.abriou@st.com>
Signed-off-by: Nicolas VANHAELEWYN <nicolas.vanhaelewyn@st.com>