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>

drivers/gpu/drm/msm/Kconfig

@@ -3,6 +3,7 @@ config DRM_MSM
 	tristate "MSM DRM"
 	depends on DRM
 	depends on ARCH_QCOM || (ARM && COMPILE_TEST)
+	select REGULATOR
 	select DRM_KMS_HELPER
 	select DRM_PANEL
 	select SHMEM

drivers/gpu/drm/msm/mdp/mdp4/mdp4_kms.c

@@ -381,6 +381,10 @@ struct msm_kms *mdp4_kms_init(struct drm_device *dev)
 	if (IS_ERR(mdp4_kms->dsi_pll_vddio))
 		mdp4_kms->dsi_pll_vddio = NULL;
 
+	/* NOTE: driver for this regulator still missing upstream.. use
+	 * _get_exclusive() and ignore the error if it does not exist
+	 * (and hope that the bootloader left it on for us)
+	 */
 	mdp4_kms->vdd = devm_regulator_get_exclusive(&pdev->dev, "vdd");
 	if (IS_ERR(mdp4_kms->vdd))
 		mdp4_kms->vdd = NULL;