drm/msm: Fix fall-through warning in msm_gem_new_impl()

Fix the following fall-through warning:

drivers/gpu/drm/msm/msm_gem.c: In function 'msm_gem_new_impl':
drivers/gpu/drm/msm/msm_gem.c:1170:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
 1170 |   if (priv->has_cached_coherent)
      |      ^
drivers/gpu/drm/msm/msm_gem.c:1173:2: note: here
 1173 |  default:
      |  ^~~~~~~

by replacing the /* fallthrough */ comment with fallthrough;

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>