drm/msm: Remove Kconfig default

Remove the default for CONFIG_DRM_MSM and let the user select the driver
manually as one does.

Additionally select QCOM_COMMAND_DB for ARCH_QCOM targets to make sure
it doesn't get missed when we need it for a6xx targets.

v2: Move from default 'm' to no default

Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>