drm/msm/sde: Update code for LTM buffer control property

Update LTM code to get buffer size from gem object directly instead of
from gem->dma_buf. The dma_buf pointer of gem object might be set to
NULL due to handle_count is reduced to 0, but the dma_buf is actually
still valid.

This change also update the log for LTM buffer control to prevent
unnecessary logs been printed out during idle power collapse and
suspend/resume.

Change-Id: I57a34f1c325c38e505b7faa2ab79fb676a7f6c72
Signed-off-by: Ping Li <pingli@codeaurora.org>