drm/amdgpu: Add bad_page_cnt_threshold to debugfs

Add bad_page_cnt_threshold to debugfs, an optional
file system used for debugging, for reporting
purposes only--it usually matches the size of
EEPROM but may be different depending on the
"bad_page_threshold" kernel module option.

The "bad_page_cnt_threshold" is a dynamically
computed value. It depends on three things: the
VRAM size; the size of the EEPROM (or the size
allocated to the RAS table therein); and the
"bad_page_threshold" module parameter. It is a
dynamically computed value, when the amdgpu module
is run, on which further parameters and logic
depend, and as such it is helpful to see the
dynamically computed value in debugfs.

Cc: Alexander Deucher <Alexander.Deucher@amd.com>
Cc: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Luben Tuikov <luben.tuikov@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>