drm/i915: Drop the broken cursor base==0 special casing

drm/i915: Drop the broken cursor base==0 special casing

The cursor code tries to treat base==0 to mean disabled. That fails
when the cursor bo gets bound at ggtt offset 0, and the user is left
looking at an invisible cursor.

We lose the disabled->disabled optimization, but that seems like
something better handled at a slightly higher level.

Cc: [email protected]
Cc: Takashi Iwai <[email protected]>
Cc: Jani Nikula <[email protected]>
Signed-off-by: Ville Syrjälä <[email protected]>
Link: http://patchwork.freedesktop.org/patch/msgid/[email protected]
Reviewed-by: Chris Wilson <[email protected]>
(cherry picked from commit 663f3122d00c0b412d429f105dca129aa8f4f094)
Signed-off-by: Jani Nikula <[email protected]>