drm/msm/sde: memory leak fix for virtual plane formats

The supported format list for virtual planes was created dynamically
when needed and in certain cases would only be freed if an error
condition was hit.  This patch fixes the memory leak and cleans up
the design for future improvements to virtual plane formats.

CRs-Fixed: 2078448
Change-Id: I46a8864372644d398758a0557bc8b8a45612b31c
Signed-off-by: Steve Cohen <cohens@codeaurora.org>