drm/scheduler: Scheduler priority fixes (v2)

Remove DRM_SCHED_PRIORITY_LOW, as it was used
in only one place.

Rename and separate by a line
DRM_SCHED_PRIORITY_MAX to DRM_SCHED_PRIORITY_COUNT
as it represents a (total) count of said
priorities and it is used as such in loops
throughout the code. (0-based indexing is the
the count number.)

Remove redundant word HIGH in priority names,
and rename *KERNEL* to *HIGH*, as it really
means that, high.

v2: Add back KERNEL and remove SW and HW,
    in lieu of a single HIGH between NORMAL and KERNEL.

Signed-off-by: Luben Tuikov <luben.tuikov@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>