drm/i915/gem: Make drop_pages() return bool

Commit e4e80625

 ("drm/i915: Change shrink ordering to use locking
around unbinding.") changed the return type to int without changing the
return values or their meaning to "0 is success". Move it back to
boolean.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Nirmoy Das <nirmoy.das@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220503061556.513175-1-lucas.demarchi@intel.com