Add preliminary support for xorg.conf "Enable" option.

Signed-off-by: Laércio de Sousa <laerciosousa@sme-mogidascruzes.sp.gov.br>
Add support for xorg.conf "Enable" option.

The driver will only try to enable a given output if this options is set
to "true" in the corresponding "Screen" section.

Signed-off-by: Laércio de Sousa <laerciosousa@sme-mogidascruzes.sp.gov.br>