gfx_common: Make gfx_init_dwm a boolean function

Also fixes a bug where inited would be set to true, even if initialization had failed. I don't think it really matters in this case, however if this changes and is called twice for some reason in the future, it won't falsely assume it's already initialized.
Merge pull request #1004 from lioncash/bool

gfx_common: Make gfx_init_dwm a boolean function