msm: kgsl: Allow users to set context priority at creation

Allow context priority to be passed in as a flag when creating
contexts. Priority is set only at context creation and cannot be
changed. Priority is used as a hint to the dispatcher when sending
commands to the GPU hardware.

Priorities are only hints. If the driver wishes, it can reject the
priority passed in. If the driver rejects the priority (if there
are too many high priorities for example), then the new context will
be created at the default priority level.

Change-Id: I12253275b7045048d59d326fbecc6446714f33a9
Signed-off-by: Carter Cooper <ccooper@codeaurora.org>