drm/msm: drop ARCH_MSM Kconfig depend

The ARCH_MSM depend is redundant with ARCH_MSM8960, so we can remove it.
Additionally, we are splitting Qualcomm MSM support into legacy (ARCH_MSM)
and multiplatform (ARCH_QCOM).  The MSM8960 with be ARCH_QCOM going forward
so dropping ARCH_MSM will work properly for the new ARCH_QCOM multiplatform
build.

Acked-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Kumar Gala <galak@codeaurora.org>