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>
diff --git a/drivers/gpu/drm/msm/Kconfig b/drivers/gpu/drm/msm/Kconfig
index 9d907c5..5b2a1ff 100644
--- a/drivers/gpu/drm/msm/Kconfig
+++ b/drivers/gpu/drm/msm/Kconfig
@@ -3,6 +3,7 @@
 	tristate "MSM DRM"
 	depends on DRM
 	depends on ARCH_QCOM || (ARM && COMPILE_TEST)
+	select REGULATOR
 	select DRM_KMS_HELPER
 	select DRM_PANEL
 	select SHMEM