platforms/drm: Compute correct cursor transform matrix

Currently, when the DRM platform uses cursor planes, the cursor on
a rotated output may be cropped because the math behind the current
cursor transform matrix is off.

In order to fix the cropping issue, this change replaces the current
cursor transform matrix with the core part of the surface-to-buffer
matrix, which was written against the wl_output spec.

BUG: 427605
CCBUG: 427060