drm/msm: select REGULATOR

Fixes a potential error, spotted by Felipe with randconfig:

-----
drivers/gpu/drm/msm/mdp/mdp4/mdp4_kms.c: In function ‘mdp4_kms_init’:
drivers/gpu/drm/msm/mdp/mdp4/mdp4_kms.c:384:2: error: implicit declaration \
        of function ‘devm_regulator_get_exclusive’ [-Werror=implicit-function-declaration]
  mdp4_kms->vdd = devm_regulator_get_exclusive(&pdev->dev, "vdd");
  ^
drivers/gpu/drm/msm/mdp/mdp4/mdp4_kms.c:384:16: error: assignment makes \
        pointer from integer without a cast [-Werror]
  mdp4_kms->vdd = devm_regulator_get_exclusive(&pdev->dev, "vdd");
                ^
-----

Also add a brief comment explaining the use of _get_exclusive()

Reported-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Rob Clark <robdclark@gmail.com>