drm/amdgpu: Check for valid number of registers to read

Do not try to allocate any amount of memory requested by the user.
Instead limit it to 128 registers. Actually the longest series of
consecutive allowed registers are 48, mmGB_TILE_MODE0-31 and
mmGB_MACROTILE_MODE0-15 (0x2644-0x2673).

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=111273


Signed-off-by: Trek <trek00@inbox.ru>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>