drm/msm/sde: dynamic layer mixer reservation for prim disp

SDE driver should not reserve multiple layer mixers for primary
display if target does not have sufficient layer mixers for
other displays. Such static reservation blocks higher resolution
display for non-primary display. This patch gets the layer mixer
requirement for primary DSI display if available. It reserves
those layer mixers dynamically for the primary display when
connector is registered. Primary layer mixer reservation falls
back to dtsi based support if connector ops are not implemented.

Change-Id: I2e7ccd0c68e1f10d458b10150c737aa0193cfec2
Signed-off-by: Nilaan Gunabalachandran <ngunabal@codeaurora.org>