drm/msm: move display and event threads to realtime priority

Display thread processes the work assigned by HAL for
screen update. Current logic selects the thread priority
to default and causes the frame drop. This patch moves
the display thread to realtime priority to process the display
work items at realtime. Event thread must follow the
display thread priority to avoid frame_pending counters
beyond 2.

Change-Id: I9154b749550cee52da1d16d22a8418676325e769
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>