drm/imagination: Fix timestamps in firmware traces

drm/imagination: Fix timestamps in firmware traces

When firmware traces are enabled, the firmware dumps 48-bit timestamps
for each trace as two 32-bit values, highest 32 bits (of which only 16
useful) first.

The driver was reassembling them the other way round i.e. interpreting
the first value in memory as the lowest 32 bits, and the second value
as the highest 32 bits (then truncated to 16 bits).

Due to this, firmware trace dumps showed very large timestamps even for
traces recorded shortly after GPU boot. The timestamps in these dumps
would also sometimes jump backwards because of the truncation.

Example trace dumped after loading the powervr module and enabling
firmware traces, where each line is commented with the timestamp value
in hexadecimal to better show both issues:

[93540092739584] : Host Sync Partition marker: 1    // 0x551300000000
[28419798597632] : GPU units deinit                 // 0x19d900000000
[28548647616512] : GPU deinit                       // 0x19f700000000

Update logic to reassemble the timestamps halves in the correct order.

Fixes: cb56cd610866 ("drm/imagination: Add firmware trace to debugfs")
Signed-off-by: Alessio Belle <alessio.belle@imgtec.com>
Reviewed-by: Matt Coster <matt.coster@imgtec.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250221-fix-fw-trace-timestamps-v1-1-dba4aeb030ca@imgtec.com
Signed-off-by: Matt Coster <matt.coster@imgtec.com>