coresight-tpdm: Optimize the useage of tpdm_has_dsb_dataset

Since the function tpdm_has_dsb_dataset will be called by TPDA
driver in subsequent patches, it is moved to the header file.
And move this judgement form the function __tpdm_{enable/disable}
to the beginning of the function tpdm_{enable/disable}_dsb.

Signed-off-by: Tao Zhang <quic_taozha@quicinc.com>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Link: https://lore.kernel.org/r/1707024641-22460-3-git-send-email-quic_taozha@quicinc.com