msm: mdss: fix null dereference when using hardware cursor

Null dereference kernel panic is seen when using hardware cursor because
buffer for cursor is not allocated before using hardware cursor API.
Allocate buffer once when API is first called to ensure buffer is
available to hold cursor image.

Change-Id: I2250c4dd93e54fa4e7122c2465fbaf1deef0fa1e
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>