ASoC: Fix kernel panic caused by snd_soc_dapm_context null pointer.

- dapm_seq_run will invoke dapm power sequence for pre-sorted list
of widgets to be powered up. The power up sequence changes will be
applied by invoking dapm_seq_run_coalesced with snd_soc_dapm_context
pointer as parameter.
- Kernel panic issue is observed during stability runs with the
above sequence caused by snd_soc_dapm_context null pointer
dereference in dapm_seq_run_coalesced.
- Fix kernel panic issue by checking for valid snd_soc_dapm_context
pointer in dapm_seq_run before invoking dapm_seq_run_coalesced.

Change-Id: Ic7238c7f0423f291e35f2045b3ea84788787253b
CRs-fixed: 336876
Signed-off-by: Jayasena Sangaraboina <jsanga@codeaurora.org>