drm/msm/mdp4: don't care about fb in crtc

Since we are configuring things via MDP4_PIPE regs in the plane, it seems
like setting the dimensions of the primary plane on the OVLP/DMA regs in
crtc is unnecessary.  This will make life easier when we want to do a
nofb modeset.

Signed-off-by: Rob Clark <robdclark@gmail.com>