drm/etnaviv: unconditionally enable debug registers

A later change will use the FE debug registers to improve GPU
progress monitoring. Instead of having to keep track of the
usage state of the debug registers and lock access to the
VIVS_HI_CLOCK_CONTROL register, statically enable debug
register access during GPU init.

The Vivante downstream driver seems to do the same thing since
a while, so it should be okay to keep access enabled. (See
gckHARDWARE_InitializeHardware in 6.4.11 downstream driver).

Many debug registers contain bogus data if clock gating is
enabled, so even if they are always accessible performance
profiling still needs to manage some prerequisites.

Reviewed-by: Christian Gmeiner <cgmeiner@igalia.com>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>