compositor/main.c: use pixman renderer by default when gl-renderer not enabled

When the gl-renderer is not enabled, weston fails to start, as it
doesn't automatically fallback to the pixman renderer, which is
always enabled.

This commit changes the drm-backend to set by default the --use-pixman
option to true when the gl-renderer is disabled (BUILD_DRM_GBM is not
defined).

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>