backends/drm: Fix buffer centering with active hardware transform

Prior to beb6cca6, the drm backend used
to specify QRect(0, 0, modeSize.width(), modeSize.height()) CRTC rect,
so we didn't have to care about buffer transforms, but now that we
compute the CRTC rect from the buffer size, we have to take the buffer
transform into account.

It fixes squashed output contents when the output is rotated 90 or 270
degrees and it uses hardware transforms.