compositor-drm: consider the best mode of the mode_list as an option

compositor-drm: consider the best mode of the mode_list as an option

This patch fixes an issue where Weston using the DRM backend, cannot start
the display. This happens in the following context:
- no video mode is set before weston starts (eg no "/dev/fb" set up)
- weston is not configured with any default video mode (nothing from
  weston.ini nor command line)
- the DRM driver provides with a list of supported modes, but none of them
  is marked as PREFERRED (which is not a usual case, but it happens)
In that case, according to the current implementation, the DRM compositor
fails to set a video mode.
This fix lets the DRM compositor selects a video mode (the best one of the
list, which is the first) from the ones provided by the driver.

Signed-off-by: Fabien Dessenne <fabien.dessenne@st.com>