drm/amdgpu: Optimize EEPROM RAS table I/O

Split functionality between read and write, which
simplifies the code and exposes areas of
optimization and more or less complexity, and take
advantage of that.

Read and write the table in one go; use a separate
stage to decode or encode the data, as opposed to
on the fly, which keeps the I2C bus busy. Use a
single read/write to read/write the table or at
most two if the number of records we're
reading/writing wraps around.

Check the check-sum of a table in EEPROM on init.

Update the checksum at the same time as when
updating the table header signature, when the
threshold was increased on boot.

Take advantage of arithmetic modulo 256, that is,
use a byte!, to greatly simplify checksum
arithmetic.

Cc: Alexander Deucher <Alexander.Deucher@amd.com>
Cc: Andrey Grodzovsky <Andrey.Grodzovsky@amd.com>
Signed-off-by: Luben Tuikov <luben.tuikov@amd.com>
Acked-by: Alexander Deucher <Alexander.Deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>