drm/amd/display: fix type of variable

Currently, the maximum value that *counter* can reach is 255, and
code at line 150: while (counter < 1000) { implies a bigger value
could be expected.

Fix this by changing the type of variable *counter* from uint8_t
to uint16_t.

Addresses-Coverity-ID: 1470030 ("Operands don't affect result")
Fixes: 2b6199a1 ("drm/amd/display: replace msleep with udelay in fbc path")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>