drm/msm/dsi: use msm_gem_kernel_put to free TX buffer

Use exiting function to free the allocated GEM object instead of
open-coding it. This has a bonus of internally calling
msm_gem_put_vaddr() to compensate for msm_gem_get_vaddr() in
msm_get_kernel_new().

Fixes: 1e29dff0 ("drm/msm: Add a common function to free kernel buffer objects")
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/562239/


Signed-off-by: Rob Clark <robdclark@chromium.org>