drm/dsi-staging: account for h_active for pixel clk calculation

DSI driver calculates the pixel clk and notifies client
about the rate to make the MDP clock rate. It should consider
only h_active to avoid bumping up MDP clock based on full
h_total. That will save power by not crossing voltage corner
 for MDP clock.

Change-Id: Ic8136ab5b75deae19ada9e0e953ffa44f53cfe00
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>