[platforms/drm] Delete buffer on all errors in present

Summary:
When returning early in DrmOutput::present() because of some error KWin
didn't delete the proposed buffer, therefore not releasing the surface
lock of the GBM buffer to the EGL surface.

This patch makes sure that on any error in present we cleanup the proposed
DrmBuffer.

Reviewers: #kwin

Subscribers: #kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D6660