drm/msm/sde: limit primary plane count to number of crtcs

DRM primary planes are meant for dedicated use by a single CRTC,
so limit the number of planes flagged as 'primary' to the max
number of CRTC's that will be created.

Change-Id: Ia0626a81d033d2c554408cda29a2e6fbe32a6667
Signed-off-by: Clarence Ip <cip@codeaurora.org>