drm: msm: reserve drm resource for early RVC

When early RVC is enabled in bootloader, the pipes on the display
which RVC is on should not be available to uers' Apps. That means,
user should not touch and allocate the pipe for other Apps before
RVC exits. For this purpose, this patch reads the reserved pipe
information from device tree, and reports pipe availability to
user from kernel side.
In additional, user's layer mixer stage will decrease by 1 to
let RVC's layer stays on top before it exits.

CRs-Fixed: 2225630
Change-Id: Id4530afb82da52aecdf5aa48c8fbd59313da236b
Signed-off-by: Guchun Chen <guchunc@codeaurora.org>