sdm: Introduce fallback mode for pluggable

If a pluggable display is being created with a mode that needs a clock
which is not supported by driver or HW fallback to 640x480 mode.

If 640x480 is not exposed by kernel, no fallback is supported.

Change-Id: Iba099df043456f6b53fdd1fa4181bec38dc91589
CRs-Fixed: 2336902
7 files changed