drm/panthor: Wait for _READY register when powering on

panthor_gpu_block_power_on() takes a register offset (rdy_reg) for the
purpose of waiting for the power transition to complete. However, a
copy/paste error converting to use the new 64 register functions
switched it to using the pwrtrans_reg register instead. Fix the function
to use the correct register.

Fixes: 4d230aa2 ("drm/panthor: Add 64-bit and poll register accessors")
Signed-off-by: Steven Price <steven.price@arm.com>
Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Link: https://lore.kernel.org/r/20250630140704.432409-1-steven.price@arm.com