drm/msm/sde: Create generic function for resource allocation

Current code has two different functions to handle the
resource allocation for continuous splash and for the
commit path. This change optimizes the code by
creating a generic function that can be used by both paths.

Change-Id: Ied62d1700125bef54cf915e87738d2273f4a83b1
Signed-off-by: Nilaan Gunabalachandran <ngunabal@codeaurora.org>