Summary:
This change hooks up the bits for changing the mode when a request to
change mode from kscreen is received. This includes:
 * setting new mode in DrmOutput
 * creating new buffers in SceneQPainter backend
 * creating new GbmSurface in SceneOpenGL backend

Room for improvement:
 * detect whether the new mode works and go back to old one if it fails

Test Plan:
Started both a QPainter/OpenGL session. Used kcmshell5 kscreen
to modify the resolution. Saw that it got applied, maximized a window to
verify it's correct. Changed back to original resolution. Verified it looked
good by maximizing a window.

Reviewers: #kwin, #plasma, subdiff, sebas

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D8398