drm: msm: remove unused variable

A recent cleanup removed the only user of the 'kms' variable in
msm_preclose(), causing a harmless compiler warning:

drivers/gpu/drm/msm/msm_drv.c: In function 'msm_preclose':
drivers/gpu/drm/msm/msm_drv.c:468:18: error: unused variable 'kms' [-Werror=unused-variable]

This removes the variable as well.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 4016260b ("drm/msm: fix bug after preclose removal")
Signed-off-by: Rob Clark <robdclark@gmail.com>