msm: adsprpc: retrieve table index directly from context ID

Currently, a linear search of the entire context table is being
done for freeing a particular entry, which is inefficient. Instead
retrieve the index already encoded in the context ID and directly
free that entry in the table. If the context ID at the retrieved
index does not match with the given ID, then fallback to searching
the entire table.

Change-Id: I22891f2686fd2f181b4d8c7f369fc24c5cfa3322
Acked-by: Thyagarajan Venkatanarayanan <venkatan@qti.qualcomm.com>
Signed-off-by: Himateja Reddy <hmreddy@codeaurora.org>