msm: kgsl: Define unique values for context flags

Use BIT to define all context related flags. This ensures
that these flags are unique and is easier to maintain. Also,
fix spelling of CTXT_FLAGS_BEING_DESTOYED to
CTXT_FLAGS_BEING_DESTROYED

Change-Id: I866c67b9c5d59d6117e31714756af3106018f9cb
Signed-off-by: Shubhraprakash Das <sadas@codeaurora.org>