drm/v3d: Enable Performance Counters before clearing them

On the Raspberry Pi 5, performance counters are not being cleared
when `v3d_perfmon_start()` is called, even though we write to the
CLR register. As a result, their values accumulate until they
overflow.

The expected behavior is for performance counters to reset to zero
at the start of a job. When the job finishes and the perfmon is
stopped, the counters should accurately reflect the values for that
specific job.

To ensure this behavior, the performance counters are now enabled
before being cleared. This allows the CLR register to function as
intended, zeroing the counter values when the job begins.

Fixes: 26a4dc29 ("drm/v3d: Expose performance counters to userspace")
Signed-off-by: Maíra Canal <mcanal@igalia.com>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241204122831.17015-1-mcanal@igalia.com