don't override m_canUseMipmaps

Summary:
Seems like this override/assignment was introduced in edb0751c.

Following that commit, 450be6a3 introduced `levels` param
so we could tell whether mipmaps are going to be used. And it seems like this assignment
hadn't been deleted in that commit.

Reviewers: #kwin, fredrik

Reviewed By: fredrik

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D11301