drm/msm: Don't allow zero sized buffer objects

Zero sized buffer objects tend to make various bits of the GEM
infrastructure complain:

 WARNING: CPU: 1 PID: 2323 at drivers/gpu/drm/drm_mm.c:389 drm_mm_insert_node_generic+0x258/0x2f0

Zero sized buffers serve no appreciable value to the user so disallow
them at create time.

Change-Id: Ic0dedbada2a0250227d7ee8c45c35dc92a826c67
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>